I forgot to add a certificate for income tax adjustment. Can I still add that? If so, how?

No, you can't add the certificate while your return is pending. Pending means that your e-filed return is on its way and that the government hasn't accepted or rejected it yet.

E-filed returns generally sit in pending status for 24–48 hours before being either accepted or rejected. 

If your return gets rejected, you'll be able to add the certificate. If your return gets accepted, you'll have to do an amended return. At that time, you'll be able to add the certificate to see if it increases your refund. 

To amend your federal return, click here.

To amend your state return, click here.