Only one relationship is allowed between activity &2 and &3

What causes this issue?

Only one relationship can exist between activities located under the
same operation.
The relationship type is End-Start. A mode linkage
using the resource and a continuous load are required.

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

System Response

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.