Postings to network & not possible without activity

What causes this issue?

Network &V1& is activity-assigned; you cannot post to it without
specifying an activity.

How to fix this error?

Check your input or enter an activity number.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.