Change of auto. post. ind. is allowed for special activity types only!!!

What causes this issue?

The activation of automatic posting is only allowed for special
activity types which are described in the documentation.
SAP cannot guarantee that the automatic posting of another activity
type will be successful!

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

If the entry is not a copy of the allowed activity type (see
documentation), please deactivate the automatic posting indicator.

Error message extract from SAP system. Copyright SAP SE.