There are a lot of articles to be found on this subject but here is a condensed procedure:

Your SQL transaction log is too big and you need to shrink it before SQL server explodes through lack of disk space:

1. Back up your database – optional but a good idea as you are about to invalidate your transaction log backup history read more…

While trying to backup a SQL database using Microsoft SQL server management studio the following error occurs:

Microsoft SQL Server Management Studio
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Backup failed for Server ‘ServerName’. (Microsoft.SqlServer.Smo) read more…