SQL claims users do not exist when they do.

I wanted to grant access to a table to a user so I exeuted this:
GRANT SELECT,UPDATE,INSERT on [MyDB].[dbo].[MyTable] TO MyUser

Which returned this message:
Cannot find the user 'MyUser', because it does not exist or you do not have permission.

I then ran this:
EXEC sp_change_users_login @Action='update_one', @UserNamePattern='MyUser',    @LoginName='MyUser';

Which returned this:
Terminating this procedure. The User name 'MyUser' is absent or invalid.

I am logged in as 'sa' and 'MyUser' is a valid SQL user.

I have checked other SQL users and they fail the same way.

What is wrong here and do I fix this?
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rwheeler23Asked:
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Brian McDonaldIT ManagerCommented:

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rwheeler23Author Commented:
EXEC sp_change_users_login 'report'  Returns no users

I just tried the GRANT command again and it worked. The difference now is most users have gone home and the load on this old server is much lighter.
Brian McDonaldIT ManagerCommented:
If they were in the system it'd probably block adjustments to their user permissions, because of active use. So that makes sense.
rwheeler23Author Commented:
A more accurate message would have been helpful.
rwheeler23Author Commented:
Thanks
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