Sounds like the external data source is referencing a different copy of the file than the one that you "bundled" with your primary file.
Should I be leaving the ClientData database file out of the runtime development since it's just a external database source?
No, it needs to be included. When part of your solution references data from a table in this file, the external data source entry tells Filemaker where to find and open that file. But when a runtime opens it and it doesn't have the needed "bind key" created during the production of the run time file, you get this error message.
Thus my post that it appears that you have an external data source reference to a copy of the file that was not made part of your runtime project.
You may need to open Manage | External Data Sources and see what you find there...
That did it Phil! I appreciate all your help. When you said "bind key" it clicked that both files would need to have it or else it would fail. Thank you again. Your always the guy to fix any solution!