Xkyer does your host allow raising the value for PHP's memory_limit ? Flarum doesn't usually need a lot but during installation, during extension installation and whenever cache is cleared additional memory can be needed to update the database and/or compile the javascript+css files.
In this particular case it seems like there's not enough memory to compile the CSS files. If you can give it a bit more memory it would help.
If you are unable to raise the memory limit, there are ways to run those specific jobs from the command line where there usually is no PHP memory limit, but it might not be as convenient as raising the memory limit.
There is no specific memory limit that you need. But the more extensions you use, the more memory might be required to compile the files.
