Have you offered to pay them for the 30 hours of time you're asking them to give up? If not, why not?

"it would take 3 weeks (15 days) for 3 hours after work." < there's probably your reason.

If you want 100% attendance, you would need to run it on-site during office hours.

As someone who has gone back to school as a working adult and also participated in company-sponsored certification classes: It's hard. It's a lot of work. Even if the company is paying for it, it's a lot to ask, especially if the employee has to give up personal time to do it. Not everyone is going to be happy about it, even if it is a benefit.

@PhilipKendall, we have not offer to pay that time, however, we are offering a reduction of daily tasks to properly accommodate the course in their schedule.

Also, these 20 people have "trust" positions (non-union workers) and most of them have senior positions.

I can't even tell if this is supposed to be serious. If someone told me now, at 35 years of age, with a toddler and pregnant wife that I should do what you are suggesting, I'd laugh in their face and tell them to get out of my office.

"This is confusing me as I don’t understand their reasons to act like that." - Have you asked them? What did they say? I'd have to agree with most guesses here, that giving up 45 hours of time unpaid (even if it is for education and therefore could be argued to be for their benefit in the long run) is a huge imposition on their precious time, but that's just guessing. The actual people you're dealing with might have a different reason.

The first thing to do when someone is acting in a way you don't understand is to ask them why, and what they need/want–politely, with the full intent of finding a mutually beneficial outcome for the both of you. Asking on an online forum is a good way to get ideas for mutually beneficial outcomes, but it's not good for finding others' motivations. Online commenters are only able to guess using the limited information that you provide, that comes from your limited perspective as a single person. This is just some friendly advice, in general :)

"so people don’t have to spend any of their resources for the classes.". Their time is a resource, it's in fact their most precious resource. There will be some who do not want to give up this time, and others who truly cannot.

Is that certification the kind where you just attend the courses and at the end you are certified, or the kind where you attend, then work on what you learned before you can have a chance of getting the certification? Some certifications require a LOT of personal work and commitment in addition to the time spend in lectures/group sessions.

I don't know what "continuous improvement" means - to me, it sounds like one of the buzzwords that I hear a new one every half year, and of the 70 of them I heard during my professional life, maybe 5 are still around, and the others died just as quickly as they appeared. I'd have no motivation at all to spend time on putting work into such a thing and getting certified until someone explains the benefits to me. If you want to raise awareness, don't get people certified; get them a 1 hour presentation that explains the thing during work hours.

I would challenge you as to whether this certification class is needed. Why does an office of people need to be certified in your defined field? Will they be teaching others this material? Or do they simply need to know a point or two to cover what you need from them? Take the material you have learned from this certification class and streamline it into an hour or two long "bare bones" PowerPoint presentation with just the essentials. Then you can present it to your team during office hours without disrupting their other work.

@Erik to clarifiy: it's the "after" that is the problem

To make a general life lesson perfectly clear: You can't get 20 people to agree on anything. Anything short of raises or bonuses, you'll never get 20 people to agree on any one thing. Even with raises, a few people will be suspicious, and will ask if extra work will be expected.

Is this even legal? A company should have no right to force people to attend courses outside working hours. I would be quite offended if my employer even asked me this. I have attended numerous trainings/courses, but those have all been during working hours. If you truly think this is important, I think you should plan for the course to happen during paid working hours. If you think that seems too costly, then that means you think the course is not worth it, and you should not promote it.

"They offer us the certification at a great discount" - just to be sure, company is paying, right? You don't expect them to pay any money in addition to doing unpaid overtime?

Education is as cheap as what udemy/pluralsight/skillshares monthly fees are.

Seriously? You don't understand that some people enjoy doing things other than work? You don't understand that some people have families? You don't understand that some people like to spend time with people other than their workmates? You don't understand that sometimes, people have commitments that aren't anything to do with work? I don't understand your lack of understanding.

From your perspective this certification is a huge benefit that the company is offering to pay for, but not everyone views it this way. If my company asked me to spend 5 hours a day of personal time learning something that I didn't see any reason for me to learn, I might get a bit cranky too.

I think OP should consider personally offering to spend 3 hours every day outside of work time helping the employees with personal commitments like babysitting, cooking dinner, cleaning the house, doing laundry etc (things they would not be able to do if they attended the course). This way, he could show his own enthusiasm and willingness to spend time outside of work, in order to inspire the employees to do the same.