I don't know all your requirements. But I guess you want "Billing Status" field to be mandatory if "A La Carte" is equal to "Yes"(Correct me if I am wrong).
In this case you need to use below formula:
IF([A La Carte]="Yes",NOT(ISBLANK([Billing Status])),TRUE)
Basic syntax for IF statement in calculated formula is:
So, try to write down your formula in above format.
Sometimes comma(,) does not work in formula (I am not sure but it is based on something language on your site). So in that case use semicolon(;) instead of comma(,).
For detailed information and ...
Make sure you've got the appropriate spaces between terms. So between any [Field] or any operator (+,*,/), and I think your parenthesis, too?
Your syntax looks correct to me. If the spaces doesn't fix the issue, try making a new field to do the calculation and call it something like NumeratorAsPercent then make your function
( NumeratorAsPercent | ...
I resolved it using below formula:
=IF(ISBLANK([Assessment]), "",IF(ISBLANK([Reassessment]), DATE(YEAR([Assessment])+3,MONTH([Assessment]),DAY([Assessment])), DATE(YEAR([Assessment])+6,MONTH([Assessment]),DAY([Assessment]))