Hello Jiten,
although the validation expression functionality reports that error, the expression is working. Can you ignore the message, activate your view and test it.
Of course you will still get a "runtime" error if the first four characters cannot be converted to an integer. Is it an option for you to split the parameter into two parameters (one for the fiscal year and one for the fiscal month, typed to the right types to be type safe)?
Regards,
Florian