G/L account & is invalid

What causes this issue?

The G/L account that you have entered is invalid.

How to fix this error?

Check whether you have entered the G/L account correctly. If necessary,
correct your entry.
If you have entered the correct G/L account, check
the validity period of the G/L account,
the cost element type of the G/L account.

Error message extract from SAP system. Copyright SAP SE.