Don't just mount the folder. Mount it as a CD-ROM ( -t cdrom ). It takes three extra keystrokes and saves you an hour of googling.

Now go play that old Phantasmagoria or Warcraft II disc. You’ve earned it. Have a different DOSBox error? Let me know in the comments below!

Or worse:

You need to tell DOSBox, "Hey, treat this like a spinning silver disc."

DOSBox sees this as a hard drive folder. It won't look for MSCDEX there.