box 12 flows automatically to the 8889 line 9 and reduces the amount you can contribute personally. only your personal contributions are entered on line 2 (line 2 does not include what's on the W-2). so if line 9 is $5000 and line 2 is more than $3000 you have an excess (line 2 is more than line 13). line 12 is the max you can contribute personally (it's basically the max allowed reduce by employer contributions on line 9). by your comments, I assume you have family HDHP and you or your spouse are eligible to make a catch-up contribution of $1000 and neither were covered by medicare at any time in 2019.
understand that the $8000 you're entering is flowing to line 2. so line 2 + line 9 will be over the max.
