If you’re getting that message, it means the program is able to access a valid data folder, but that the database file mentioned in the message (either the .DBF or the .DBT or both) is missing from that data folder. If you right-click on the Essential Forms shortcut, click "Properties" from the drop-down, in the "Shortcut" tab, put the cursor in the "Target" window outside of the last quote (if you have one) and put a (space) /Q at the end of the "target". Click “Ok” and then “Apply.” For example (on mine) it looks like this:

"C:\Program Files (x86)\CEB\Essential Forms\EFWin32.exe" /Q

Launch the program (with the command line switch /Q) and you'll get the opportunity to select a data folder, and it should show you where it's currently pointing in that window.
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The file(s) might have been moved, renamed, or deleted, so if you can't find them, they'll need to restore from backup.

Once you have determined the data path and whether the file referred to in the error is actually missing, you’ll want to remove the /Q off the “target” so your not prompted to choose the data directory every time you launch the program. Right-click on the Essential Forms shortcut, click "Properties" from the drop-down, in the "Shortcut" tab, in the "Target" window delete the /Q at the end of the "target". Click “Ok” and “Apply.”

Otherwise, if the program opens and you’re missing client data you might need to restore the latest Essential Forms Data folder backup.