Hello,
1) Could you, please, provide us with temporary WP Dashboard access to your site to check the mentioned error while you edit it? Also, provide us with server error logs to get more information about error.
2) Where exactly do you want to add the Subject field in the form layout? Above the Message area?
3) Error message appears below the input field https://prnt.sc/tprgun Where do you want to display it?
Regards