Allowed posting periods: &

What causes this issue?

The posting date is not within one of the allowed posting periods.
This can be due to one of two reasons:
The correct current period has not been set in the system.
For the system, the first of the specified periods is the current
period. At the beginning of a new period, your system administrator
must change the current period in the material master record using the
function "Close period". This has not yet been done.
You have entered the wrong date in the field "Posting date".

How to fix this error?

Check the posting date and correct it if necessary. If your entry is
correct, inform your system administrator that the "period closing"
procedure has not yet been implemented.

Error message extract from SAP system. Copyright SAP SE.