Cause
This error occurs when an updating method has been called for an operand value that is the result of a compound expression; for example, consider the following statement.
(s1 & s2).firstCharToUpper;
As (s1 & s2) does not identify a storage location that can be updated, this syntax is invalid.
Action
Change your application code to replace the compound expression with a simple expression that can act as the receiver of an updating method. In some cases, it may be possible to change the method being called from updating to non-updating (if it does not actually update the receiver).