Field & is already defined as a characteristic -> Change your entry

What causes this issue?

The environment of a structure contains a field catalog which contains
all the fields of all generated (and saved) data structures. If a field
is contained once in the field catalog, it and its attributes can no
longer be changed.

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?

If you wish to use a field from the field catalog, you need to select
it using the appropriate menu option. If you wish to define a new
field, you must give it a name which is not already contained in the
field catalog.

Error message extract from SAP system. Copyright SAP SE.