The psychologist who did this test said you cannot substitute any subtests- you can only substitute for example if the child couldn't pick up the block for block design.
Yet I've seen that alot of people have substitutions for this test. What does that mean?

Substitutions are supposed to occur for reasons of serious interruption (fire alarms in the middle of a timed test), disability (block design with someone without arms), or prior planned substitution. I do often see scores calculated with just the best available from among standard and supplemental tests which, in theory, shouldn't occur. Most kids would score better if you picked best 10/12 or best 6 or 8 or whatever, than if you picked 6/6 or 10/10 subtests.

Just MHO and all that -- maybe someone with more background than I have will comment.

According to our educational psychologist, it's reasonable to substitute subtests when a significant discrepancy occurs. For example, if a child received 15-19s on most subtests, gets a 15 on Symbol Search but gets an 8 on Coding. It would be reasonable to give the Cancellation subtest to determine if the low score on coding was a fluke or indicated a weakness in that area.

Agree with PP, but then it would have to be noted in the report. In my dd's case there was some subtest in which the tester did not get the sense she had really tried hard enough, she was getting sick, hadn't slept much, etc. Tester decided to give a supplemental test a couple of days later. In her report she indicates her regularly calculated scores and in the written part, she explains the extra subtest, how her scores would be affected, etc and then goes on to explain that it is not clear to the tester which of those scores better reflect her true abilities.

Some testers only go for a substitution if they themselves (tester) messed up pretty badly in giving a certain subtest.

Our tester said absolutely that type of substitution is not allowed. It seems reasonable to me though. Just our luck, we get the completely by the book tester.