SQL Always on availability with failover and mixed authentication
Either create the login on instances hosting the replicas specifying the same SID as on the instance hosting the primary: 


  To Resolve an Orphaned User 
  
  In the master database, use the CREATE LOGIN statement with the SID option to recreate a missing login,
  providing the SID of the database user obtained in the previous
  section: 

CREATE LOGIN &lt;login_name&gt;   
WITH PASSWORD = '&lt;use_a_strong_password_here&gt;',  
SID = &lt;SID&gt;;



From: Troubleshoot Orphaned Users (SQL Server)

Or use a Contained Database User:

CREATE USER user_name WITH PASSWORD = 'strong_password';