Maybe because I'm combining Xbox Live Gold and Game Pass to Ultimate Game Pass? It specifically gave an error saying it wasn't able to extend the subscription that far.
Ugh, I went over the combined 36 months (only by 2 months) and it won't let me convert. I figured I would just lose the two months, but nope. Contacted support and they weren't able to figure out a solution but are working on it and are going to email me back.