I am only logged in on “1” machine, and it gives me a link to “Manage Activations” that goes to a page where I can NOT manage activations. I even at one time setup a NEW account just to get past this, now it’s getting VERY frustrating because I can’t use either and can’t find any information on how to actually “Manage Activations”. Addtionally there is NO support number to call, for the several hundred they charge a year to tens of thousands of users you would think a support number is common sense… How do I “Manage Activations”
Each version of SketchUp you are signed in under counts as another activation, even if they are on the same computer. You could open SketchUp 2019 and 2020, and make sure to sign out of them before closing them. Then 2021 should have no problems.
If you do need to deauthorize devices to get a fresh start, follow what this article says: