I can't pass this challenge Process Design Without Limits Learn About Object Design-Time Limits. I have below error message. Can anypne please help.

Challenge Not yet complete... here's what's wrong: 
Changing the owner of the Account record failed to update the 'Account_Owner_Name_Process__c' field for the Case object . Make sure that the process is correct and that it is activated.https://trailhead.salesforce.com/modules/process-design-without-limits/units/process-design-without-limits-object

I have read in the communities guide that a contact can log into different communities with same username. But eloborate description not provided on the same. Does anyone have this requirement or if anyone implemented the same can you kindly guide me to have a knowledge on it how to go about it.