This error may be many different issues. Make sure you are not trying to edit an archived company. You can tell this by looking at the Sage 50 program title bar. Make sure it does not say Archived (company name), you cannot edit an archived company. If you are not...
I/O error in file Sage 50 Accounting US Edition
Prior to initiating any data recovery procedures, it is imperative to create a recent backup of your company’s data. It is strongly advised not to merely execute the data integrity checks. These processes are quite stringent and may eliminate any data they deem to be incorrect. This could result in the loss of transactions, customer/vendor information, prior period balances, and other crucial data.

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Unable to determine security privileges Sage 50/Peachtree
Sage 50 and Peachtree Accounting Software - Unable to determine security privileges. Please retry later. This is probably due to another user temporarily blocking access to critical files.This error is caused by damaged .dat files. It has nothing to do with User...
At least one damaged transaction has been found in account [Account ID] in the period beginning
This Sage 50 error message notifies you that there are damaged transactions. If the beginning period is within the current 2 open years, Data Verification should fix it. If the period is outside of your open 2 years your data will have to be repaired using our Sage 50...