First, search for the file "wpoptins-style.php" and see if it exists. If it doesn't, there's your problem.

But if it does, then chances are it is not in the same folder as the file wpoptins_squeeze1.php. So the "include("wpoptins-style.php")" line in your wpoptins_squeeze1.phpisn't pointing to that file. So you need to find which folder that file is in and update the include line. If you need help, feel free to PM me.