I have two objects A and B, object A records related to B records,which i want display the count of that related records?

I am beginner :  Not able to solve Trailhead challange for-Developer Beginner :Process Automation-Automate Basic Business Processes with Process Builder
"
Create a process to update child record when the parent is updated.
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.
NOTE: You may have to deactivate the validation rule for the Contacts object (created from a previous challenge) in order to complete this challenge."

 What I had tried 
Deleted validation rule on contact  from previous challange.
 Using Process builder created new Process with below details
Object: Account
Criteria:'Update Address' with condidtion

Field: selected shipping street, city, state,postalcode and country
Operator: Ischanged
Type: boolean
value: true

Condition:
Any condition met (OR)

 For Immediate action
Action name: Mailing address change
Records: Account.contacts
Criteria for Updating Records:No criteria—just update the records!

Set new field values for the records you update:

Field: selected Mailing street, city, state,postalcode and country
Type: Reference
value:  selected shipping street, city, state,postalcode and country

 after this saved and activated. So Once I check challange got below error:
"Challenge Not yet complete... here's what's wrong:
An update to an account record failed to update the mailing address fields of all child contact records. Make sure that the process is correct and that it is activated."

So Please help.





 

I'm experiencing a strange problem that cropped up yesterday morning.  I am suddenly getting a 'page not found' error when trying to access my community in my sandbox org.  I have not changed any of the custom URL settings in weeks and was accessing and using the sandbox community the very night before without issue.

Sandbox: https://sandbox-hirebotics.cs51.force.com/ (<-- page not found error)

Production:  https://robots.hirebotics.com (<-- works fine)

After three phone calls / go-to-meetings with salesforce support they can't figure out what's going on.  Their latest guess is that my product custom URL in sandbox (which is copied into sandbox upon a refresh, which I haven't done in a week) is set to "in development".  But they can't tell me why that's a problem now nor how to change that, assuming that's the problem.  They suggested I post here.  See below.


Now, I also tried spinning out another developers sandbox from my working production org and I'm seeing the exact same problem accessing the login page hosted from this new dev org.

Note that the error page below is being served from my org as the email us link connects to my email address.


Also, both my sandbox and dev sandbox are hosted on CS51, if that's relevant.

Any idea what's going on?