The issue seems to be that the game launched reports an error, either it was launched incorrectly (i.e. not starting the right executable) or the game executable needs additional parameters when starting.
Since for DOS there is no one rule for all games you can see what is the actual error is by using the +Start Dosbox shortcut, run a cd <game folder> command and try to launch the game manually by typing the command (game.exe or game.bat). See what is the error reported and try to fix it - maybe a file is missing or you're launching the wrong command to start the game, game needs a CD to work, game expects to be run from a certain folder, etc.
You have to try game by game to see how the game works, but once you find it, you can create a game.conf file which can use used to launch the game. In the game.conf file you enter:
[autoexec]
command 1
command 2
# other commands.
# last command is just exiting the DOS env
exit
where you replace command 1, 2, etc with the commands needed to launch the game.
For instance, to lauch Little Big Adventure 2 by changing to the lba2 folder and executing LBA2.EXE, my .conf file would contain:
[autoexec]
# map C: to the `pc` roms folder
mount c /home/pi/RetroPie/roms/pc/
# simulate a CD drive on D: from the `.dat` file
imgmount d "/home/pi/RetroPie/roms/lba2/LBA2.DAT" -t iso
c:
cls
cd LBA2
LBA2.EXE
exit
Place the .conf file in the same folder as the rest of the PC games (pc) and restart EmulationStation.
