The Tascam mixers uses the .bwf type of wav-files. It should be able to read an ordinary .wav-file, but there also exist RF64 versions of .wav-files that can be larger than 4gb and I don’t think such a file could be read by the mixer.
So you might need to check first what type of file you’re trying to import:
an ordinary .wav-file,
a .bwf-file or
a RF64-file.
You could try to open your file in a wave editing program to find out what it is and also try to convert the file if need be.