EC2 instance connectivity error

Hello.

I want to directly connect ec2 instance from AWS Dashboard.

Perhaps you are trying to connect to EC2 using Instance Connect.
The following documents list the prerequisites for use, so please make sure you have met all of them.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-connect-set-up.html

how to find my SSH user name login as for connecting with putty.

EC2 uses the public key authentication method for SSH connections by default.
So it is possible to SSH from putty, etc. using the private key corresponding to the key pair set at EC2 startup.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-key-pairs.html
Note that the user name will change depending on the operating system you are using.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-to-linux-instance.html#connection-prereqs-get-info-about-instance
To use password authentication for SSH users, the EC2 sshd settings must be changed.

Can I connect and using aws SES SMTP and brevo SMTP both at the same time?

Depends on the application hosted on EC2.
For example, Application A could use Amazon SES to send email and Application B could use Brevo to send email.