Creating a new world gives me the error of "Errors in currently selected datapacks prevented the world from loading. You can either try to load it with only the vanilla data pack ("safe mode") or go back to the title screen and fix it manually" with Pixelmon 9.0.0. Safe Mode doesn't work. I have tested it with Forge+JEI which works, but Forge+Pixelmon doesn't work. Deselecting any Data Packs results in "Data pack validation failed" even with the Default and Forge's datapacks selected, I have to have Pixelmon completely disabled before it will work. Does not crash so it doesn't generate a crash report, but MultiMC does give me the error that the log file has gone over 100,000 lines.

Found the issue. MultiMC was defaulting to Java version 18, I checked "Java installation" and hit Auto-detect and chose the recommended version of 1.8.0_3333. Working now.

Good to know! Good on ya figuring it out :)