Try changing your Phone Country Code to Canada, and then back to US -- in some cases that seems to allow things to proceed.
If you are still having trouble signing up for Managed Payments, there are some good troubleshooting tips to be found in this thread:
https://community.ebay.com/t5/Payments/Managed-Payments-Registration-Walkthrough/m-p/31092219#M79735
Common issues involve making sure your eBay account type matches your checking account type (personal account vs. business account), making sure your name is spelled exactly the same on both accounts including any middle name or initial, and making sure you are using the correct routing number for your checking account. I believe that you may need to use a street address rather than a PO Box as well.
