Please I need assistance my form have been showing me this error message

Welcome to the community, @mutication! The error means there is an if() in the calculation for ${grade} that does not follow the required syntax:

if(expression, then, else) — exactly 3 arguments.

This community post discussed previously should also help you solve on how an if() is used:

Besides, you could also always validate your XLSForm through this online validator to checkout for syntax issues.