Msg 3023, Level 16, State 2 Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed.

Question : Executing the following  Database shrinkfile actibity and getting the error message

use myDatabase
DBCC SHRINKFILE (N'myDatabase_log' , 0, TRUNCATEONLY)

Msg 3023, Level 16, State 2, Line 4
Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed.

Answer: Check if there are any other Backup jobs running on the same database  which could be interfering with the command you are attempting to execute .   A typical scenario is where another Backup Job could be executing and is for some reason failing. 

A good clue about the nature of the issue is the error message text - Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized.

In that situation , identify the other process - which is either a Backup, filemanipulation or encryption change  and KILL IT.  A good tool to use is  sp_WhoIsActive utility for SQL Server troubleshooting.   

Then execute the DBCC SHRINKFILE (N'myDatabase_log' , 0, TRUNCATEONLY)