What causes this issue?

The multi-variable &V1&[&V2&] ( "Multiple individual values")
should be replaced with the value of variable &V3&[ &V4&]. But &V4& is
a formula variable. Therefore the system
does not support this replacement.

System Response

Variable &V2& cannot be used.

How to fix this error?

Check the variables &V2& and &V4&.

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.