Testing biome setting in 1.16 Release Candidate 1 does not change my biome. Not in a flat world, nor a regular world. No error messages were produced.

The command I tried to run was /script scan -12 55 81 -12 55 81 -75 61 145 set_biome(_, 'plains').

well, its just the documentation is lacking.

set biome is inconsistent as biomes are in the game. For the OW and the end, you have to set it at Y=0 ad affects entire column. In the nether is block based.

need to update docs on that first

Nice. I just switched the y value to 0 and it works. Thanks!