thanks for your detailed reply. now let me start from the beginning to describe my current situation. i have VS 2015 professional version 14.0.25 update 3. i also have WPI installed and inside it, it mentioned that the "Microsoft Office Developer tools update 2 for VS 2015 " is already installed... so what should i do now ? shold i uninstall SPO cleint components and re-install them ?

yes that might help. Also, can you check if the Microsoft.SharePoint.Client dlls are present at C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI location ?

now inside the control panel i can not find SPO Client components SDK .. i can only see a program named "sharepoint client components" and not "sharepoint online client components".. for the location C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI yes the Microsoft.SharePoint.Client is already there...

so what do u think ? now i already run this commadn Install-Package Microsoft.SharePointOnline.CSOM inside my project but it did not fix the problem..

@Guatam now i try downloading the sdk from microsoft.com/en-in/download/details.aspx?id=42038 but when i try to install the sharepointclientcomponents_16-6906-1200_x64-en-us.msi i got an error that another version of this product is already installed !!

hmm strange, can you check if the Microsoft.SharePoint.Client and related folders are present in the C:\Windows\Microsoft.NET\assembly\GAC_MSIL path, something like this and this? Also, yes would recommend that you uninstall the SharePoint client components and reinstall using the download link mentioned above

so now to understand the case exactly so there are 2 programs one named SharePoint client components while the other named SharePoint online client components?? as in my case inside the control panel i only have "SharePoint client components" ?? so should i uninstall it then reinstall the SPO client component ?

No, there will be only one, SharePoint client components. Uninstall that and reinstall using the link i mentioned. Apologies, if i created any confusion.

ok i got your point. but keeping in mind that the version for the "Sharepoint online components" is 16.0.6518.1200 inside the control panel ,, but not sure why visual studio is trying to load the one with version 15.. anyway will re-install as suggested from ur end.. thanks. unfortunately i do not have enough reputation score to like ur value replies..

now i uninstall the "SharePoint client components" from control panel, then i download the newer version from microsoft.com/en-in/download/details.aspx?id=42038 then i install sharepointclientcomponents_16-6906-1200_x64-en-us, after that i try to create a new SP-hosted add-in, but when i try to add ERE i got the exact same error as the on mentioned inside my original question, i also try creating a new provider-hosted add-in, but when i try to add RER i got the same error!! not sure why VS is trying to load the version 15, although i mentioned that i need it for SP online!!

second question, now from the error message i am receiving when trying to add a new RER, it mentioned that "it can not connect to the site url or the current user does not have permission on the site"?? now i am running VS using my account, while the SP add-in url is for our customer, so my username does not have permission to access the url... could this be the problem? if so then how i can fix it? should i login to the VS using the customer account admin username ??? i am totally lost...

Looks like we made some progress i guess. Can you enter a valid url ? After that, i guess it will ask you for user name and password. So, can you enter the details of a user who has access to that site collection ? Also, usually, its something like user.name@tenant.onmicrosoft.com

@GautamSheth now now finally i got it working, i install the "SharePoint Server 2013 Client Components SDK" restart VS , then i can run my remote ER.

after installing this component, when want to create a new SP-addin i will be prompted to enter username and password....then i run the RER project and seems every thing is working well... so "SharePoint Server 2013 Client Components SDK" was the missing component !! although i can not understand wyy i do need an SDK for SP 2013, for developing SP-addin for office 365!!

lol :D was about to post the same :)

the SharePoint Online Client Components SDK is dependent on SharePoint Server 2013 SDK for some weird reason

@GautamSheth welcome to sharepoint :)

oh man :(

@GautamSheth if you can update your answer so i can accept it ..

sure, give me a few mins. Out to fetch milk and frame something meaningful. Hope, this SharePoint dev life has a purpose :D