An Azure relational database service.
If you are performing a backup to URL on an Azure SQL VM please use a classic storage account or v1 storage account. Folllow other instructions here.
In addition, consider COMPRESSION, MAXTRANSFERSIZE, BLOCKSIZE, and multiple URL arguments when backing up large databases as explained here.
If you are using Microsoft SQL Server 2012, 2014 or 2016 please consider applying SQL Server updates mentioned on KB4017023 support article.