Backup Failed - The transaction log for database 'SampleDB' is full

Backup failed due to log file size reached limit 

Solution 1:

1. You can perform shrink database according to guide in below link:

Please refer How to Shrink Database Transaction Log

Solution2:

Increase the size limit of a database log file
1. Connect to Database use Microsoft SQL Server Management Studio
2. Expand Databases, right-click the database to increase, and then click Properties.
3. In Database Properties, select the Files page.
4. To increase the size of an existing file, increase the value in the Initial Size (MB) column for the file.
5. Click OK.