help, what should I do?

Without any specific info about your project we can't really tell you much. This could be a single effect used somewhere like the many instances of Twixtor you seem to be using according to your comp names. It's probably competing with AE for GPU resources on your very limited old Mac. As such, the most sensible thing to do would be to turn off GPU acceleration in AE itself plus also switch Twixtor to software-based rendering, not GPU. Anything beyond that will require more info.

Mylenium

Just follow the instructions in the pop-up - basically there's too many calls on your graphics card and it can't handle what's being asked - the pop-up is suggesting switching to Software render only. AE has been using your graphics card to boost processing, but that can be tricky and sometimes it's simpler not to use it.