Cant add users to sharepoint

[OctoTech]

I just created a new Sharepoint, and added users to it but none are there. I go to add them manually, and I get an error showing "Couldnt Add Users". Im not sure why. These are local users, not external. 

What can I do to fix this?

[#2]

I found a work around, but any idea why this happens this way?

I was able to add them via this method.
https://answers.microsoft.com/en-us/msoffice/forum/all/not-able-to-add-a-user-as-a-member-in-any/c6c...