Cannot perform a backup or restore operation within a transaction.BACKUP DATABASE is terminating abnormally.

You are trying to take a Database Backup and you might end up with below error:

"Msg 3021, Level 16, State 0, Line 1
Cannot perform a backup or restore operation within a transaction.
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally."

The error clearly says you are running Backup operation under a transaction, sample code below

QUERY:

BEGIN TRAN
GO
USE master
GO
BACKUP DATABASE [TEST] TO DISK = 'C:\Test.bak'
GO

Sample Error Screenshot:

This is because backup command is a Serialized operation and you cannot run under this a transaction. Just run the backup command outside transaction as shown below and issue Rollback command to make sure the transaction is no longer valid.

Corrected Statement:

USE master
GO
BACKUP DATABASE [TEST] TO DISK = 'C:\temp\Test.bak'
GO

Sample Success Screenshot: