Unfortunately, I'm a bit pressed for time so I can't give you an exhaustive answer as I often try to do.

My guess is that you're hitting up against the PHP memory and/or filesize upload limits. Without checking, I can't guide you explicitly to what you'll need to change but I can give you some hints.

Firstly, TurnKey v15.x appliances are based on Debian 9/Stretch (the upcoming v16.x will be based on Debian 10/Buster). So if you find info that relates to Debian Stretch, it should also apply to TurnKey v15.x. Info re earlier or later versions of Debian will often also generally apply. As Ubuntu is also based on Debian, info re Ubuntu may also apply, but be a little careful there as Ubuntu and Debian are completely binary compatible (TurnKey is binary compatible with Debian). Although, so long as you avoid installing Ubuntu software, you should generally find info re Ubuntu generally useful.

Re PHP, if I'm right, the file you'll need to update will be the php.ini file. On TUrnKey there are 2, one for CLI (commandline php) and the other for Apache (webserver php). The one you'll need to edit it the Apache one. You should be able to find the appropriate file like this:

find /etc -type f -name "php.ini"

That should return both files and you'll be able to tell which is which by the path of them.

Once you have made the required changes, then you'll also need to restart Apache. Do that like this:

systemctl restart apache2

Sorry that's a bit scattered and not as immediately or explicitly helpful as I'd ideally like, but hopefully it's enough info to get you going in the right direction...

Regardless, please post back with what you tried and how you got on. If you continue to have struggles, if you could try to explicitly describe step-by-step how I can recreate the issue you're hitting, I'm happy to double check myself. And if I can recreate the issue, then there's a good chance I can work out how to fix it! :)