That’s the part of the error message that tells you that it is looking for “ReLU” as a function name, but you gave it “Activation”. Your code would actually work, I think (has the same effect), but the test was written to check specifically for that function name. It is frequently the case in programming that there can be more than one way to write the code that works. Normally they only judge by the output of your code, but unfortunately in this case they are checking for the actual function name.
For the new error, about OperatorNotAllowedInGraph, that’s harder to interpret. What it’s telling you is that you did not call the ReLU layer correctly. In this second part of the exercise, we’re doing the Functional API, not the Sequential API. Here’s a thread that is about the same mistake that you made. Please have a look and that should explain it.