Syntax error: Variable name &1 is not allowed

What causes this issue?

Variable name '&V1&' has a format which is not allowed. Variable names
must fulfil the following requirements:
The name must begin with a letter, and it can only contain letters,
digits, and an underscore.
The name cannot be identical to a keyword or to the name of an installed
function.
The name cannot be longer than 20 letters.

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?

Alter the variable name so that it corresponds to the requirements
mentioned above.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.