What tends to work for me is to write out one line a time and see if there are errors in the line before moving on. Notice all the lines in your calculation are in one execution group (>).
For what you post, the error occurs in the 3rd line of the big execution goup where you write something with "z". As @gkokovidis points out, there appears to be a missing "mutiplication" sign between the "z" and the "x". While I don't think you need to convert to Maple input mode, (I use 2D input all the time), I do agree it should say "z * x(t)"
The rest suggests you have copied from some other source. My experience is that it is best to understand each line well before moving on to the next. There are some incredibly smart and gifted folks here who are likely to help you out before I have a chance to look at the rest.
(P.S. Thank you for the complement.)