Hello, most of the details can be found in this issue report for Netherite Beacons here: IAmSneak/netherite-beacons#1 but I will give a TL;DR

Netherite Beacons 1.2.1, Beacon Overhaul 1.8.2, Fabric, 1.20.1
The log is in the other issue report and shows a mixin conflict, obviously both mods modify beacons so that's what it's complaining about, check the report for more details!

As an aside, I really like the concept behind Beacon Overhaul, but the lack of configuration does make it harder to work with from the community standpoint. If you can get these mods working together though, you won't even need to add your own configuration, because Netherite Beacons has an easy to use config file. It would save you some trouble and allow people to fit their use case without needing to worry about datapacks or unchangeable features. If not though, thanks for taking the time to look and for your work on the mod!