@Bawolff I notice you have been active on this topic. I am one week new to Mediawiki and have spent four days to try and fix the "Lua error at line 159: Missing JsonConfig extension..." with now effect.
Did you find a solution? (In simple language please 😃)
Basically you need to setup extension:JsonConfig with the right config (which i dont think is documented anywhere) in order to have the template be able to fetch the data: namespace page. This is super complex to do, and i don't think anyone has written a guide. I wouldn't really reccomend this as a first week activity as it will be really frustrating. It might be easier to try copying a really old version of the template before jsonconfig was a thing.
Sorry, i know that wasn't particularly helpful.
@Bawolff Thank you for the quick response.
I have decided to go for now with templates that I can build with Extension:Page Forms.
Cheers!