iphone videos not playing on mac (MOV files)

Mojave IS macOS 10.14.

Going from El Capitan to Mojave is an OS upGRADE, not update. You will need to full Mojave installer.

Make sure to check if your mac supports it, do not attempt the upgrade with having a full backup.

As to why some videos do not play, they are probably using HEVC, whereas the other videos are in H264.

Move the old application to the trash. In the App Store go to your Purchased page to download the application not to the FCP page. Click the button in the lower left for your account and you’re purchased applications.

Still not updated. Sorry.

Try this

macOS Mojave 10.14

‭+1 (800) 692-7753‬.

Contact Apple for support and service - Apple Support for numbers outside the US.

You might also be able to connect to chat depending on where you are.

What version of FCP? How did you pass the video to the external drive, or did you pass it to the Mac first? How did you do that? How is the external drive connected and formatted?

As Luis said, if this is recent iPhone material it’s likely to be in HEVC, which requires at least High Sierra 10.13 and FCP 10.4.

You have downloaded the 10.14.1 updater. You need the full OS installer. One way is to use the MDS software https://bitbucket.org/twocanoes/macdeploystick/downloads/

Not at all. That computer can run the current OS and anything in between.