@Keith Thank you for reaching out to us, If I understand correctly you are trying to deploy Sentinel All-in-One from here - https://github.com/Azure/Azure-Sentinel/blob/master/Tools/Sentinel-All-In-One/README.md while doing the same subscription field shows blank?
what happens when you login to portal.azure.com using the same account? does it show Azure Subscriptions linked to this account?
As per the above GitHub doc having Azure Subscription is pre-req to deploy Microsoft Sentinel All-in-One.
Reference: https://www.youtube.com/watch?v=AZQ07n1O-Ds
Let me know if you have any questions, feel free to post back.