Edit mask is not correctly defined

What causes this issue?

The edit mask you entered is not valid. When creating the edit mask,
please be aware of the following rules:
The following characters are allowed for edit masks:
The special symbols allowed in Customizing (as editing characters). At
the present time these are the symbols "&V1&".
An "X" as placeholder for alpha-numeric characters.
A "0" as placeholder for numerical characters.
One of the allowed special symbols "&V1&" must be in the first place of
the edit mask.
Spaces are not permitted in the edit mask.

How to fix this error?

Please correct your entries.

Error message extract from SAP system. Copyright SAP SE.