Looks like many other people have run into this exact error before [Programming Assignment: Convolutional Neural Networks: Application]
You’ll help yourself and save a lot of time if you learn how to use the forum search to your advantage. For example, I found the above thread quickly after searching on “Z1 =“
It may not be obvious from the error stack, but information in this thread [Tips for troubles with Sequential and Functional API syntax] may also prove useful both here and going forward in the Specialization.