Select a valid line

What causes this issue?

You want to carry out a function, but have not positioned the cursor on
a line that will enable the system to perform this function.

System Response

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?

To carry out an item-related function, you must first position the
cursor on the relevant item.

Error message extract from SAP system. Copyright SAP SE.