Lifting from hg38 to hg37
BED format can refer to the &quot;.txt&quot; file format that stores chromosomes and positions of genomic features as well as some other things like name. bed format can also refer to the plink format, which belongs to bed, bim, fam trio. The plink bed format stores all the genetic variants, but the program needs to refer to bim file for their positions.
Genome liftover obviously only needs the coordination of each SNPs, and it does not need individual genotypes at all. In fact, the bed format in PLINK does not provide any useful information. Knowing this means that providing the plink bed file is wrong. And this is pretty clear from the error message.
Moreover, since the three plink file always needs to be used together, most programs refer to them as plink file format. So whenever people say BED format, it is the txt file format.