I just created an object based on the custom object and fields - when I try to save the API name as Trail___c, as described, it will not allow me to save the field with the double underscore.

Please help me i am getting this error

The challenge is:

You've been given a requirement to keep Contact addresses in sync with the Account they belong to. Use Process Builder to create a new process that updates all child Contact addresses when the address of the Account record is updated. This process:Can have any name.
Must be activated.
Must update Contact mailing address fields (Street, City, State, Post Code, Country) when the parent Account shipping address field values are updated.

I start with Accounts when records is created or edited.
Filter criteria where accounts shipping address (street, city, state, zip code) is changed (used "or" logic).

I am stuck on the next step "add action".  I started with action type "Update Records".
On objects, I can't seem to change Accounts to Contacts.  Why?