Dennis: You can use pan_created panels and click on their buttons. A background image or a second panel is not required. So your problem likely has a different reason. Using buttons is described in the lite-C Workshops; I suggest that you use the panel from the workshops in order to find out if it's something with your images or maybe something else in your script. If you can't find out what's wrong, you can contact the support and send them your project.

Oldschooli: Don't be sloppy with your code. Wrong syntax can cause trouble with any future update, even if it does not throw error messages in the current version. I also see no reason why you should intentionally use wrong syntax. So better go and fix your 36 buttons by replacing |= with =.