How do I add another MX record to Google Domains that uses a lower priority? When I try to add the lower priority MX record it gives me this error, "Record already in use". 
I followed Google's instruction here, 
Issue Creating Two MX Records with Different Priority Using Google Domains
mx-record
Best Answer
You should click the Edit button in the MX record that Google considers valid, click '+' and add the second MX record