Issue with using autofill for an IF/IFs activity

[s.cas]

I've not had this issue before, but I'm trying to complete some Excel IF questions and I just began a lesson teaching on 'nesting' IF functions - after typing in formula and selecting autofill, the first 2 cells are correct, however the remaining cells are not, the syntax changes so that the formula computes cells which have no contents- or essentially the wrong cells. please can someone help me to figure out this issue, I'm sure it's quite simple but since I'm a complete newcomer to excel, I am having some issues.

[TMS]

The references to cells A17 to B20 should be absolute, for example, $A$17