When running a Period Update You may see the above warning.

The message means that your Customer Aging report does not match the G/L balance for the controlling Accounts Receivable account.

Normally, the total of your customer aging should match your G/L A/R balance. Similarly the total of your vendor aging should match your G/L A/P balance. If one or both do not, then you will be asked if you wish to proceed with the Period Update. This is simply informative.

There is no real harm in proceeding. However, if you are using the General Ledger you should attempt to harmonize the Aging Report with the A/R balance, either by making a manual entry into the G/L or by finding the incorrect open item in Sales Orders or Payments/Credits.