Why are some iPhones unable to play MP4s?

MP4 is a wrapper. The audio and video inside are individually coded using codecs which come in many varieties. It could be that your app is using a proprietary one or just one not included in Apple's decoding software. Try using the free VLC to play the movie if this is not a protected file.

Since you are using Windows it could be you are indeed using a codec that is not Apple-friendly.

I also invite you to browse the vendor's own support page https://support.techsmith.com/hc/en-us#kb

Some more details:

• https://www.captureguide.com/iphone-video-format/

Here are what Apple recommends for media creation, when submitting that media to Apple for distribution:

• https://help.apple.com/itc/videoaudioassetguide/en.lproj/static.html

If you’re picking one, then HEVC / H.265 works with iPhone 7 and later, iOS 11 and later, and macOS 10.13 and later, and it is very efficient:

• Using HEIF or HEVC media on Apple devices - Apple Support