Job reports warning:
SQL VSS Writer is missing: databases will be backed up in crash-consistent state and transaction log processing will be skipped
Running the command vssadmin list writers shows that 'SqlServerWriter' is not active.
There are different causes of the issue:
Reported causes\solutions to fix the SQL VSS Writer not be present on a SQL Server:
select '#' + name +'#' from sys.databases where name like '% %'
Microsoft documentation states that the SQL Writer service must run as Local System.