[leve]

My books says

Different structural isomers are not possible. The H2O can bond at any one of the four equivalent sites with NH3 in the other three sites.

I do not understand the reasoning that isomers are not possible. Can anyone explain?

[Arkcon]

OK, draw for us two different isomers.  You can do it in paint, and attach, or try to get a stick-figure in wordpad.  If you have two, be sure they're unique, and not just mirror images of the same one.