What happens when you try to open the file in Wavelab? Nothing? An error message?
If it’s m4a, I think it can either be AAC or ALAC (apple lossless), and I believe if it’s ALAC it can only be opened in 32bit (not 64bit) Wavelab, but only after changing the extension to .alac . Unless something’s changed with Wavelab. AAC I think should open in either Wavelab 32 or 64.
You could try opening the file in iTunes and see Info /file properties /file type to see if it’s Apple Lossless.
Or you can just look at the file size. if it’s big it’s probably ALAC, if it’s small it’s probably AAC.
Did you try any other m4a files?