[20:58:23] [main/WARN] [FML]: Found FMLCorePluginContainsFMLMod marker in lootr-1.12.2-0.6.2.jar. This is not recommended, @mods should be in a separate jar from the coremod.

This isn't an error but a warning that, at this point in time, isn't particularly relevant. Lootr uses Mixin Booter in 1.12 as noted on the CurseForge page; please ensure that you have installed Mixin Booter.