why does this code result in an error?

Question

https://code.sololearn.com/c62SM969qLxd/?ref=app
Need help guys
Thanks

Answer 1

You have first time called d. For this reason cannot be written
d = a + ... d
d = a + b + c

Answer 2

Thank you so much!!!

Answer 3

Now it's fixed 😀

https://code.sololearn.com/cAmnV2Q0342Q/?ref=app