Why AWS charge me by the month when I choose annual subscription?

I subscribed to skill builder, since I already have the cloud practitioner certification (I used the free resources), but to study and prepare for SAA all the options are subscription only. Should I cancel my subscription and look in other place ?

Can you share more info which subscription you mean? AWS services are mostly charged on-demand, you pay for what you use, and when you open new account, you have some services included in the Free Tier model where you run and test resources for free. If you start using AWS on a regular basis for example for production, there are payment models for some usage types like EC2 instances (Reserved Instances or Savings Plans) where you can make an annual commitment and save costs on your EC2 bills. And there are annual subscriptions in some cases for AWS Marketplace products (partner solutions) which I wouldn't believe you should subscibe to when you're just learning and testing...

If you say you are charged $29 monthly, this sounds like a Developer support plan: https://aws.amazon.com/premiumsupport/pricing/ - if you only use AWS for learning and testing, you will most likely not need a support plan, as you will likely not need to reach out to Permium Support for any issues with your services - you can still submit support cases to Customer Service (account & billing support) for free without any paid plan. You can cancel your support plan any time from your account.