Most often reason for the problem you have described is that users don't enter the registration info correctly.
Please note that client installations are free, no need to enter licenses there.
I kindly ask you to enter your registration info on the Server PC again and pay special attention to several important points:
1. Make sure that there are no excessive spaces before/after your registration name/code when entering them into the corresponding fields. Those might have been introduced by the mail client you use.
2. There should only be letters, digits and dashes in your code. We recommend to execute Copy and Paste operations when entering your registration info.
3. It often happens that e-mail client breaks the string into 2 rows or introduces excessive gaps. Please make sure that the registration code you enter is a single sting of letters, digits and dashes.