Problems with database creation triggers

Why do I insert a non-existent customer number , It doesn't output the results I want , namely “ Customer does not exist , Can't insert ”, When inserting an existing customer number , my orders surface , Failed to insert ？
The following is the title

This is the code that I created the trigger （ I think there's a problem , But I can't figure out how to change it ）

Here is the problem when I insert a non-existent customer number