You can find lots of threads about this error by searching for “multiple inbound nodes”. Here’s another recent thread that gives some links.
The first thing to try is to make sure your notebook is in a consistent state:
Kernel -> Restart and Clear Output
Cell -> Run All
Then check if the error still occurs.
Also note that we’re not supposed to share solution code in a public way. If the mentors can’t figure out how to help without seeing your code, there are private ways to accomplish that. No harm is done, since I just edited your post to remove the source code.