Cause
This error occurs if your application code assigns a value to a compound expression, for example:
a + b := c;
As a+b does not identify a storage location to which a value is assigned, this syntax is invalid.
This error is also returned if you supply a compound expression as the value of an io or output parameter. This is because a value is effectively assigned from the called method back to the corresponding parameter in the calling method for parameters of these types.
Action
Change your application code to replace the compound expression with a simple expressive that can act as the target of an assignment.
In the case of method parameters, it may be possible to resolve the error by changing the parameter type from io or output to constant (the default) or input.