You phrased it as a legal question. It goes against the rules.

If they wanted you to sign that then it should have been done when all the paperwork was completed as you resigned. In my opinion it is too late so don't sign. but IANAL.

@SolarMike I don't think anyone can be forced to sign a contract as they or after they quit. Slavery is illegal, you can't refuse somebody's resignation just because they won't sign a new contract. The contract should only have be signed as a condition of getting the position in the first place or in return for something.

@user1666620 isn't that what I said "If they wanted you to sign that then it should have been done when all the paperwork was completed as you resigned."?

@SolarMike "If they wanted you to sign that then it should have been done when all the paperwork was completed as you resigned." > this means to my eyes that new contracts are signed as part of the resignation process. The time to sign contracts which govern resignation procedures should be before the person gets the job.

If your boss has a gun in his hand then he can force you to sign it, however the signature would be void because it was done under duress. The only way my boss could force me legally is to offer lots of money to my wife for signing :-)

How is this "sit out the non compete" thing going. Are you on gardening leave, getting paid by the old employer? Are you not getting paid at all?

Does this answer your question? Being asked to sign a "termination certificate" 6 months after resigning

I am currently on gardening leave until the end of my notice period. So I am still getting paid my salary. After this there will be an unpaid non compete

This sounds like a pretty simple exit declaration that doesn't bind you to anything new, unless you had a lawsuit or claim in mind. Is there really a problem to solve here?

@TymoteuszPaul: yes, although it depends how aggressive the former employer is, and how serious a competitor the new one is. One tactic if you join a competitor is temporarily for the remainder of the non-compete period, to work in a dept unrelated to your previous job (e.g. IT, logistics or whatever), so noone could claim you were applying your knowledge. But anyway as to this document, it's not in OP's interest to sign it. Unless they negotiate compensation in return, and even then possibly not.

"They offer nothing in return for signing this document." I believe you've already figured this out for yourself :)

If it is genuinely the case that the document offers you nothing in return (which will depend on a careful analysis of the wording) and it is not executed as a deed, then it is void for lack of consideration and is not a contract. But that would be a discussion for law stackexchange not workplace.

My rule is to never sign anything unless I am getting something worthwhile in return. IMHO, too many people sign stuff (especially at medical offices) just because they are asked to sign.

Everyone: answer in answers, not in comments. Comments are for suggesting improvements to the question or asking for clarification.

talk to a lawyer

Are they offering to pay for a lawyer to remove the document? Agreements not to sue can be valid, but only if you get legal advice first. Many employers will offer a so-called "settlement agreement" and also pay for you to hire a lawyer to get it properly confirmed. But that's more common in the case of involuntary termination.