You cannot enter an activity type for standard value &

What causes this issue?

You are attempting to enter an activity type for standard value &V1&
or
the activity type was copied from the work center.
An activity type for a standard value
<ZH>may</> not be maintained if the rule for maintaining the standard
value in the work center is <ZH>0</> (may not be entered)
or
<ZH>should</> not be maintained if the rule for maintaining the standard
value is <ZH>1</>(should not be entered).

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?

Change your entry.
If the activity type was copied from the work center, inconsistencies
occur between the rule for maintaining standard value &V1&, the
activity type and your reference indicator in your work center.
Remove any inconsistencies in the work center, as follows:
Delete the activity type
or
deselect the reference indicator for the activity type
or
change the rule for maintenance for the standard value to "3" or "4".

Error message extract from SAP system. Copyright SAP SE.