When I click SAVE for step 71, it shows a URL Error

Sorry you're stuck but there's almost always a pretty easy fix! Do NOT try to start all over again, uninstall Max's Pitch Package, etc. That can make it worse. There's a 95% chance the same snag/fix is listed & answered in the Help FAQ. But if you can't find it there, post your problem here.
Make sure you post in the right category below & be sure to include a screencap showing the issue!
I have seen that occur with another student. There is a bug within the Salesforce Platform. Luckily all the steps complete before Step 71 are saved, Salesforce glitched when regarding you back to the home page. You can continue by clicking "switch to lighting experience" to return to the Salesforce Platform. Just click the Setup gear and Setup next to your profile icon to continue with Step 72.