Persistent Severity 016 alerts: "Change Tracking autocleanup is blocked on side table"
It seems to be a known issue and has already been recognized by Microsoft. You may read resolution as below:

Resolution 


  A fix for this issue is included in the following updates
  for SQL Server:
  
  Cumulative Update 10 for SQL Server 2017
  
  Cumulative Update 3 for SQL Server 2016 Service Pack 2
  
  Cumulative Update 1 for SQL Server 2014 Service Pack 3 
  
  With this fix, you will not see continuous messages any longer. The
  frequency of the error 22123 messages, if any, will not be more than
  once every 30 minutes. To completely eliminate these messages, enable
  trace flag 8293.


Listed are the version which has this issues: 


  
  SQL Server 2017 Developer
  SQL Server 2017 Enterprise
  SQL Server 2017 Enterprise Core
  SQL Server 2017 Standard Windows
  SQL Server 2016 Developer
  SQL Server 2016 Enterprise
  SQL Server 2016 Enterprise Core
  SQL Server 2016 Standard
  SQL Server 2014 Developer
  SQL Server 2014 Enterprise
  SQL Server 2014 Enterprise Core
  SQL Server 2014 Standard
  


Please read more from link.

Hope above helps.