A number of people made good suggestions, and I will try to address them all.
If the folder ends with .A .M .P .W or .X the folder refuses to add with drag/drop, all other letters are fine.
I tried moving the folder to the root directory and also tried moving the folder to C: just in case it was a problem with the path, and when I tried todrag/drop the R.E.M folder into my music library I still get the same error.
If I rename it to REM as suggested, the folder add fine, which indicates that it's an issue with the folder name, not the folder itself.
I also checked my Windows Settings and the extension .M is not associated with any program.
I also tried renaming my Jimmy Barnes folder to Jimmy Barnes.M and adding to my music folder and it causes the same error.
Just in case there was a problem with my copy of MusicBee (corrupted settings or library files causing the problem,) I just finished uninstalling MusicBee, deleting the library files and reinstalling it, and when I drag/drop the R.E.M folder into my MusicBee Music Library, I get the same error:
MusicBee v3.3.7310.31925D (Win10.0), 22 Jan 2020 7:28:
System.UnauthorizedAccessException: Access is denied. [D:\Music\R.E.M]
at #=zCf4VQ_fbhtH95rz2gNmFAbQ=.#=zQpNJpAzXUl5a.#=zu5xWrFY=.#=zgLNDeXw9Itw0(#=zlkE2wlw28gwaYlXF4g== #=zXJ5hmWc=, FileShare #=zCILa2$Q=, FileOptions #=zRbT7IJA=, Boolean #=zmFaAsvwdppoG)
at #=zs$v0QJMGgz_3qcC8MBYx$3TbhWLM.#=zgLNDeXw9Itw0(#=zlkE2wlw28gwaYlXF4g== #=zXJ5hmWc=, FileOptions #=zRbT7IJA=)
at #=zZZwzh3uXkPVnA4Lk5hhVhMs=.#=zgLNDeXw9Itw0(#=zlkE2wlw28gwaYlXF4g== #=zXJ5hmWc=, FileOptions #=zRbT7IJA=)
at #=z2UFdtdloStNYpwV6JA_AIIc=.#=z9wapuoBZRDyQ(#=zfSkwXAQ= #=zw$5Jazo=)
at #=z2UFdtdloStNYpwV6JA_AIIc=.#=zfJ0pBtI=()
at #=zcSUiT7FfjPAJ1lCP26kaFcE=.#=qsQHnGzY8$r7D68J7errB$kayMjbufN5$UAwktucMwWE=._Lambda$__0()