Hello,
How I would go about designing an If-Then-Else statement that determines whether the points variable is outside the range of 9 to 51. If the variable holds a value outside this range it should display "Invalid points." Otherwise, it should display "Valid points." ?

You could do it like this:

If points<9
Then "Invalid Points"
Else if points > 51
Then "Invalid points"
Else "Valid Points"

Or this:

If AND( points>=9, points <=51) "Valid points"
else "Invalid points"