Maybe try either OPEN "BT.BAS" or BLOAD "BT.BAS" (but BLOAD is specifically for binaries; the file may well be plain text).

You might also try leaving the .BAS out of the filename, and again trying all of OPEN, LOAD, and BLOAD.

My knowledge of Apples is very limited, but I would try the obvious to me steps of just hammering out a simple hello world basic program, saving it and trying to reload it. If that works or doesn't work it tells you which direction to go.

Also a thing about the basic file you are trying to load, if it has been on this apple disk and loaded off this apple disk exclusively, then it's maybe simple age/defect related file corruption. If it is something that has come from off the net.... well there's several ways systems can mess up "ascii" standard files by messing with the character returns and line feeds, combining or separating or replacing, to make something look formatted right on a PC vs apple vs unix etc etc, so you may end up with a file that is not correctly formatted for your machine if it's been shuffled around the net, copied and pasted etc etc. Also some while back, "endian-ness" was important in file transfer and could mess up some files when going between architectures, so that might have messed up a file from an Apple machine POV depending on how it was moved in the past. Maybe irrelevant but factors to look into if you are not sure of the file's provenance.