Hi,
Azure Plan is Pay-as-you-go, so you are fine using that. You need to Reactivate your subscription since it is disabled.
If you are careful, you should be able to keep your costs low (<$10) when using your Azure subscription for learning AZ-900. The cost will depend on how much you practice and which specific resources you choose to create and for how long.
Biggest single recommendation I can give you is to always Delete any resource groups immediately after you are finished practicing. When you are working, set an alarm on your phone for say, 1 hour in the future, to remind you to delete resource groups. Always verify that the groups have been successfully deleted because in rare cases there will be an error deleting resources inside of a group.
If you create a VM, check Run with Azure Spot discount, select a size that has low hourly charge (<$0.02/hour), for hard disk choose Standard HDD, and select small image (30GB) wherever possible for the OS image.
ALWAYS sign in to the portal at least once a day and check accrued charges for your subscription. Set a recurring reminder on your phone or calendar.
Learn how to set up alerts so that you will receive an email if charges go over certain thresholds like $10/$20/$30/$50/etc. This is just an extra "in case you make mistake" method to let you know costs are going higher than expected.
If you have doubts as to how much something will cost, post a question here in Q & A before you create it to get clarification. Learning how pricing works for commonly used resources is most important thing when getting started with Azure/other major clouds.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP