Field '&3' contains an unexpected indicator: '&4'

What causes this issue?

According to upload method &V2&, the file being read, &V1&, includes
the field '&V3&'. This field is an indicator that is either activated
(on) or deactivated (off).
In the current record, this field has the value '&V4&', instead of the
expected values ' ' or 'X'.

System Response

The non-blank indicator is replaced by value 'X'. Thus, it is
interpreted as being activated.

How to fix this error?

In the file to be read, set the field '&V3&' to one of the expected
values: ' ' or 'X'.

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.