No variable allowed before '=' when setting a property

What causes this issue?

The specified syntax contains errors. The following syntax is
permitted:

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Methods
<method name> <export variable 1>, <export variable 2>, ... <export
variable n>
or
<method name> = <variable>
or
<object>.<object method> = <variable>
Setting or retrieving object attributes
<value 1> = <value 2>

Error message extract from SAP system. Copyright SAP SE.