Set the ExportForm to something that the browser can display. The default is "WL" (wolfram language), which the browser cannot interpret. You can use an image format instead, like PNG, JPEG, etc. Below, I use "PNG"
CloudDeploy[FormFunction[{"first"->"Number","second"->"Number"}, Plot[1+3*x,{x,0,3}]&, "PNG",
AppearanceRules -> <| "Title"->"linear function", "Description"-> "two numbers"|>]]
