SAP Note 13310 - Form for account assignment manual.

Symptom:

The customer wishes to allocate a separate form for account assignment
information, accountants notes and the like, but this isn't possible.
The user can access an overview of the forms defined in the system by
selecting the menu sequence 'Format -> Form', but cannot then select a
form from this list.
Instead he receives the message: 'Function code cannot be selected'.

Cause and prerequisites

Reallocating forms is not directly supported for these object types.

G/L account additional texts belong to the type "SKA1" or "SKB1",
customer texts to the type "KNA1" or KNB1" (or the allocation to form
and style is in table TTXOB).

Solution
Maintain table TTXOB with transaction SM31 and enter the required form
there.

Key word: TTXOB