Remote harden of transaction 'CREATE INDEX' - when moving an index to a different filegroup
As Kin implied in the comments, this is a problem with communication between the primary and secondary nodes of your sync-mode availability group.

There are a number of different causes for this.  It's possible that the availability group failed over in the middle of the operation (which would cause the transaction to be rolled back and then that error occurs).

More generally, the error message:


  Remote harden of transaction 'CREATE INDEX' (ID 0x0000000082a3e3fc 0000:8d8f7911) started at Aug 14 2018 6:14PM in database 'cola' at LSN (2599205:10500:111) failed


Means that the transaction log record associated with log sequence number 2599205:10500:111 either


couldn't be sent to the secondary, or
wasn't acknowledged by the secondary


You should be able to find clues regarding what is causing this communication failure in 


the cluster log (see here for how to get that), 
the SQL Server error log (look for messages about availability group role changes), and 
the AlwaysOn_health extended event session.