Sounds like a file has gotten corrupted. Has the system gone down while users are active or an update running (power failure, OS crash, user forced shutdown)?
What the system is reporting is that when it went to delete or modify a key for a record the expected key table entry wasn't found.
In the file there are key table for the primary and all alternate keys that point to the record. When a record is changed that will impact an alternate key the system will find the old entry in the key tables that pointed to the record; remove it; and insert a new key table entry. The message appears if, for some unknown reason, it cannot find the old key table entry.
For example if you changed a client name from "Pet Sitters" to "Happy Dogs" and the system had an alternate key on client names, it would remove the key table entry for "Pet Sitters" which would have pointed to the record, and inserted a new "Happy Dogs" key table entry. The message will occur if it could not find the "Pet Sitters" entry which should have existed.
Technically when the message occurs the specific instance has been resolved however it does indicate something suspicious has occurred causing the file key tables to be out of sync which can occur when sections of the file that we pending update to the physical disk were not flushed when the system failed.
To fix, open and lock the file then issue a KEYED LOAD directive against the file. This will rebuild the key tables from the records.
Be sure to back the file first on the off chance the system cannot fully recover the data.
Also, if possible, do an application integrity check as while PxPlus defers key table updates until after the records are updated (thus the keys can usually be rebuilt from the data records), there is no guarantee that the OS flushed the data records to disc.