Small update. I figured a workaround for that error, in UEFA Champions League/Ranking Rules/Ranking rules info I edited the last row increasing the teams from 2 to 23 which brings the total to exactly 88.
I also added disciplinary and fixture rules to the continental rules, and made inactive the Youth League since it kept saying me it couldn't create stage 0 for the UEFA Youth League (although I'd like to add the competition too, just need to figure out what do I need to do).
I verified the rules and for the first time I had no errors, so I tested them ingame (I also set start year 2021 so I didn't have to sim to 2024/25) and everything seem to work flawessly and correctly, and that edit to the ranking rules info don't seem to affect the competition. I checked every round and everything seems correct, I'm going to simulate until the end of the season to make sure everything is correct. Huge milestone for me.
The only weird thing I noticed is the UEFA nation coefficient, for some reason they're mixed up despite them being in the correct order in the editor. Perhaps it may have something to do with the fact I only edited the Champions League? I'm gonna add and edit the other competitions too.
Still some work to do on the UCL, I need to add some offset dates and some alternative dates for the World Cup and I need to add a host nation pool for the final. I'd also like to calculate the complete prize pool from the actual champions league and my edited UCL since the round of 16 is no longer a thing and those are good money and I'd like to redistribute them among the teams in the earlier stages. We're talking about 75 millions of € as appearance money for the 16 teams qualified for the R16