HTML in iFrame giving Not Secure Error on HTTPS

Hello, so i have been making a inquiry form for my workplace and it seems all finished, or at least use able to the point it will redirect to the email on the computer and allow them to send the information.

Upon testing on different devices i have ran into the problem that its coming up with a “Not Secure” popup when trying to be sent via IPhone. The sites registered as being HTTPS in the url but the lock icon is missing replaced with an I symbol.

i have left the code below in this link in case its something wrong with my code. If anyone has any information that would be great as i cant seem to find anything useful online.