Why I can not send the welcoming email to my clients?

Because you have not validate your account yet. To do that:

1. Sign in to your reseller console.
2. From the left menu, click on Branding.
3. Under the Email section, click on the Validate email button.
4. Once you do that, an email from Amazon will be sent your email. Click on the link in it and follow the steps to verify your email.
5. Now, you can resend the welcoming email to the customers by going to → Accounts → Actions → Resend welcome email.