Mods used:
1 - iceandfire-2.1.13-1.20.1-beta-4
2 - alexscaves-2.0.2
3 - citadel-2.6.0-1.20.1 & citadel-2.5.7-1.20.1

When using citadel 2.6.0 the mod Ice and Fire requires citadel version between 2.4.1 and 2.5.99 (inclusive). But when using citadel lower than 2.6.0 (2.5.7 in my case) the mod Alex's Caves requires citadel version 2.6.0 or above

try to downgrade alex's cave version