Two possible reasons:
1. Due to Jenkins' bug, we cannot get value of previous radio block, so I don't know you're choosing Create new Resource Group or Use existing Resource Group. The workaround in the plugin is guessing by whether you have typed something in Create new Resource Group. So if there is something in Create new Resource Group, the Use existing storage account will always take content in Create new as dependency. If you're using Use existing Resource Group, please make sure to clean content in Create new Resource Group.
Use existing storage account will match your type of storage type, so please make sure the storage account is either Standard_LRS or Premium_LRS.