Server Error 401: Unauthorized

I added a feature Allow add in Lookup Table, but I get an error when adding a new item Server Error 401: Unauthorized

What needs to be done to remove this error?

Make sure the user has Add permission for the Lookup Tables, read Security Setup in the help file.

I have all administrator rights. In a seperate table, I can add a field, but I cannot do it in Lookup Table.

The information you provided is not sufficient to test. (Using the demo project cannot reproduce the problem.) If you are a registered user, you should use your privilege and send your files to support: https://phpmaker.dev/support.php.

Is the demo project stripped down in some features?

According to https://phpmaker.dev/demo.php:

Note: This demonstration uses some extensions (see https://phpmaker.dev/doc/extension.htm) which are available for registered user only.