This error may present itself in several ways including when entering in a new charge into Transaction Entry.
To fix, try finding a previous good backup and copying the data dictionary files (i.e., mwddf.add, mwddf.ai, mwddf.am) from that backup into dataset. Then run Repair Practice data in the MediUtils utility (commonly found at: c:\medisoft\bin\mediutils). Alternatively, running the Copy Data utility (also found in Mediutils) may also fix the issue.