G/L account must be marked as ready for input

What causes this issue?

A reconciliation account which is entered when you enter a document
needs the marking "Reconciliation account is ready for input" in the
master record. In this case, this requirement is not met.

System Response

The entered account is not allowed.

How to fix this error?

Enter another account number or see to it that the master record of
the account is changed.

Error message extract from SAP system. Copyright SAP SE.