So 299+5% and/or 299+(5%) to figure the sales tax does not work at all, it just says "Your equation has an error. Please correct it." and no hint to what the er

So 299+5% and/or 299+(5%) to figure the sales tax does not work at all, it just says "Your equation has an error. Please correct it." and no hint to what the error is. Sorry, not ready for prime time