K2 does not work with Joomla 4 beta. because, well as the name suggests, it's beta. And constantly changing as it's been evident.

K2 will be released for Joomla 4 a few weeks after Joomla version 4.0.0 "stable" is released.

If you're working on a project that you need to ship really soon, go with Joomla 3.x.

Either way, K2 is a breeze to move between Joomla versions, which is why it supports technically 3 platforms already (Joomla 1.5, 2.5 & 3.x). Not many other extensions do that...

GD is not enabled in your PHP setup. Enable it and the gallery will work. As for the error, the code that breaks is where server errors should be raised and apparently it's changed. I'll make sure to adapt the code accordingly.

But you'll need GD in any case, otherwise the plugin won't work at all.

Great.