Have you created AT&T (symbol T) in your security list before trying to add it to the account?
Also it's somewhat unlikely that the ESPP shs would have the same characteristics as the stock. Dbl-check that
You cannot add a new security to Quicken when that security already exists in Quicken.
Your Add Shares (or Buy Shares) should refer to the existing AT&T Inc security already in Quicken.