Backup is getting failed with following error
The error message "Not enough server storage is available to process this command" is a Windows error message that can be extremely misleading.  The error message indicates there is not enough RAM available to complete the operation.  

It is highly likely that Windows is very low on free memory - check the SQL Server "max server memory" setting to ensure there is a reasonable amount of memory left for the operating system.  

This command shows how much memory SQL Server is configured to use:

EXEC sys.sp_configure 'max server memory (MB)';


See What is a deterministic method for evaluating a sensible buffer pool size? for details about what this value should be. 

If you see this error, open Windows Task Manager and check how much free memory Windows has.