Looking at AmiDock's Readme, there is no "d" at the end of "interlace"... did you add it here on the forum only? ;-)
I guess this should work:
Assign >NIL: DOCK: programs:wb/amidock/dock/interlace/maindock/
Btw AmiDock is unfortunately a very old piece of software, especially the version on Aminet (I know that Gary Knight has maintained it some time after this 1.3 version, and he sent new versions to registered users (I was one of them). I don't know if he's still reachable nowadays...). In fact, ToolManager did almost the same thing and had a lot more success and improvements, but didn't work on 1.x systems.