Hi Amy,
Instead of When, you'll have to use Where, but, one thing I've learned working on If statements including Where and Calculation contexts is, try to create few variables for those Where clauses and then use them in the If statement or else, they won't work fine.
Create the below variables:
v_BIO-157_Yes = [Enrollment Start Date] Where ([Enrolled Course Name] = "BIO-157" and [Passing Grade]="Yes")
v_BIO-263 = [Enrollment Start Date] Where ([Enrolled Course Name] = "BIO-263")
And then, create that If statement, try that and let us know what happens.
Thanks,
Mahboob Mohammed