Hello, I am currently a Custodian at the Animal Jam Art Wiki. I was editing the page there called Tehbestotakuevah and one day, the editor would only let me use the Source mode since it contains "complex code". I have not added any features that should contain complex code other than columns, but literally, 99% of all pages on the wiki include columns and you can use Classic editor on them. I think my account may just be having issues. If someone can help fix this or tell me the reason behind this, it would be greatly appreciated since I don't know anything about coding.
The editor may be defaulting to source mode because of the sheer amount of excess wikitext and deprecated tags on that page. I suggest you take a look at Help:Galleries, Slideshows, and Sliders to get an idea on how to better represent your wiki's images, and I also think that splitting each artwork/section up into it's own page or infobox would be beneficial as well.
The page is just over 900 lines long, and can be efficiently condensed into about a third of that, making it easier to manage and edit in the future