Your entry in field &1 is incomplete or contains invalid values

What causes this issue?

You did not make an entry in a required entry field, or you entered an
invalid value in an input field.

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?

Check your entries.
For information on valid entries, refer to the documentation on
maintaining TRESN reservations or the online help on the V_TRESN
maintenance transaction.

Error message extract from SAP system. Copyright SAP SE.