Enter a posting key first of all

What causes this issue?

The entry facility for the account number is initiated via matchcode
search. Separate matchcodes exist for every account type (customers,
vendors, G/L accounts, fixed assets). The matchcodes which are suitable
can in this case only be determined by means of the posting key.

System Response

The entry facility can only be supplied after entering the posting key.

How to fix this error?

Enter the required posting key before you use the possible entries
pushbutton for the account number.

Error message extract from SAP system. Copyright SAP SE.