SQL Server Agent - Service Failed to Start (Access is Denied)
I hope you are using Configuration Manager to change the Service account.Using the service.msc applet does not grant required permission

Refer the link : https://social.msdn.microsoft.com/Forums/sqlserver/en-US/975c86b9-3622-418a-8e19-6720a51d7549/access-denied-starting-sql-server-agent?forum=sqlgetstarted

Other way could be to add the Service account in SQL Server as sysadmin and then configure it to run under the required service account.