Hi @ChaosDaemon and welcome to the Manjaro community.
My guess is that you may not have the AUR configured in pamac-manager
(the Pamac GUI version that ‘Add/Remove Software’ actually launches).
Please see:
Without specific information such as an actual error message and related logs, for example, there is not much else to be offered.
As a new user, please take some time to familiarise yourself with Forum requirements; in particular, the many ways to use the forum effectively. The following links will help you achieve that.
- [How to] Request Support
- [How to] Find Error Logs
- [How to] Find System Information
- [How to] Post Screenshots and Links
- [How to] Post Command Output and File Content as Formatted Text
- The Tutorials category, generally
- Strict Forum Rules and Guidelines
And last, but not least, the Stable Update Announcements, which you should check frequently for important update related information.
I hope this helps. Cheers.