Multiple line expression not permitted

What causes this issue?

You have entered the multi-line expression &V1& for the agent
assignment at this step.

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

System Response

Multi-line expressions cannnot be processed by the system at present.

How to fix this error?

Please enter a single-line expression with reference to a DDIC field
behind which the domain HROBJEC_14 is situated.

Error message extract from SAP system. Copyright SAP SE.