Selected Answer
You havent actually nested the IF functions.
Im on mobile now so i cant test one for you but here is our tutorial on nesting if functions. Check that out and it should explain everything.
Update
(back on my computer now)
Your formula is not nested correctly but also will fail to return the correct numbers since you don't check if the number is within a range. You check if it is greater than 200 and then greater than 450 etc, but if the number is 2000, it will return true for every IF statement that is checked against it. As such, I added some AND functions to check for a range and nested the IF statements.
=IF(AND((H17-I17)>="200",(H17-I17)<450),"pm1",IF(AND((H17-I17)>="450",(H17-I17)<950),"pm2",IF(AND((H17-I17)>="950",(H17-I17)<1950),"pm3",IF((H17-I17)>"1950","pm4","0"))))
In reality though, you should be using a VLOOKUP function for a range of numbers for this.