Storage accounts not showing up when creating Azure Storage Credential
I see the same thing.  I suspect that the SSMS Maintenance Plan UI in SSMS for generating an SQL Server Credential to use with Azure Storage is out-of-date, or simply broken.
I would use the built-in (and newer) Managed Backup to Azure feature instead of the Maintenance Plan in the first place.
If you do want to use the Maintenace Plan, you'll need to create the credential yourself, per the docs here.  The tricky part (and the bit SSMS was trying to save you from) is generating the Shared Access Signature, which needs to be done in Powershell, or with the Azure Storage Explorer (not in the Azure Portal).