Firstly, our support has been slow lately due to issues updating to 1.20 but announcements in the discord server have been made notifying everyone about our schedules. The errors you posted were regarding issues merging with ItemsAdder, and were errors thrown by their plugin.
We are compatible with ItemsAdder, however, and you can find merge instructions here (
https://itemsadder.devs.beer/compatibility-with-other-plugins/compatible/space)
Secondly, flags are suggested by the community. There are 195 countries in the world and even more flags than that, supplying models and textures for every single flag is an astronomical undertaking and so I simply add the flags that are requested, nothing more. This also helps reduce resource pack size to help on lower-end computers.
If you'd still like to resolve your issues, please message me directly, my discord is "olliez4"