Hi, why do I keep gettnig this error in the challenge even though it clearly exists: - What am I missing...thanks

I cannot complete the challenge in my playground because in the previous module where Knowledge was enabled, the Enable Lightning Knowledge  setting was defaulted to checked, and no instructions specifically indicated to make sure it was unchecked. Now, the steps in the next module cannot be completed. What should I do to move forward?

Help Needed.

Challenge Not yet complete... here's what's wrong: 
The 'Google Info' custom button was not found. Make sure that it is a Detail Page Button (not link) and that its added to the page layout. If you are still having problems, ensure that you are NOT using Record types for the Contact object. This challenge will not work with Record types so you will need to sign up for a new DE org and use that instead for this module.

Trailhead Module 3:  Create a formula field that determines the number of days between today and the last activity date for a case's account.
Your support team has asked for improved visibility on account activity level at the time they’re helping with customer issues. Specifically, when they’re looking at a case, they’d like to see an at-a-glance view of the number of days since the case’s related account was last active. Create the formula using these requirements.The formula should be on the Case object.
The formula should be of return type Number.
The formula should be named 'Days Since Last Update' and have a resulting API Name of 'Days_Since_Last_Update__c'.
The formula should return the number of days between the account’s Last Activity Date and today.

I created the following:

I created a new custom formula field with return type number and called Days Since Last Update.
I created this formula: Account.LastActivityDate - TODAY()

I get this error message:

Challenge not yet complete... here's what's wrong: 
The 'Days_Since_Last_Update__c' formula field did not return the correct number of days between an Account’s Last Activity Date and today

Can you please assist what I did wrong?  I'm not sure if my formula is even correct with the challenge.  Please help!

Thank you.