@ peppi
I’ve simply installed the plugin and it just work.
What is wrote here is true : https://thecartpress.com/docs/getting-started/minimum-requirements/
At the present I use the latest WordPress and the Weaver II theme + another 15 plugins.
I used the Cartpress with other themes, too in the past without problems.
If I want to blame something that would be the way of configuration.
MY ADVICE: IF ONE’S IQ IS not high enough (required average IQ) to deal with the plugin config then don’t install it.
Peppi, is the “Hello Dolly” plugin working on your WP install?