Posting key & not defined for account type 'K'

What causes this issue?

You have entered a posting key that is not allowed for the account type
"Supplier".

How to fix this error?

Change the posting key.
If you do not know the correct posting key, you can position the cursor
on the input field and press F4. You will then obtain a list of all
posting keys from which you can choose the correct posting key directly.

Error message extract from SAP system. Copyright SAP SE.