Data source definition is invalid and is deleted

What causes this issue?

When creating the screen, it was determined that the data source is
invalid in the following lines of the data flow:
Target element:,,&V1&
Priority:,,&V2&
Component:,,&V3&
Possible causes:
Constant is too long (> 100 characters)
Constant of the wrong type
Unknown source element
Unknown system field
...

System Response

The data source is deleted.

How to fix this error?

Enter a valid data source for this line.

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.