I cant login to sftp using my private key on my instance?

There are various reasons you might be unable to connect to your instance - this page offers some solutions: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/TroubleshootingInstancesConnecting.html

Do you get any specific error message? It's worth trying to connect through SSH in verbose mode (ssh -vvv) to see if that gives any more information.

By default you won't be able to SFTP to an EC2 machine. There are many articles on the Internet on how to set this up. I went through a few and this one caught my eye - https://medium.com/@clopez.lopez/enabling-a-sftp-on-ec2-instance-with-ami2-linux-82eae613c429

Disclaimer: I have not tried the instructions on this article myself and I or AWS do not endorse it in any way. You should try it out yourself and see if it works for you

If you want a fully managed SFTP solution on AWS, please take a look at https://aws.amazon.com/aws-transfer-family/

Detailed documentation here - https://docs.aws.amazon.com/transfer/latest/userguide/what-is-aws-transfer-family.html