Apple isn’t making many of its tools newly available on older versions, and there’s no easy means to get older versions of the apps.
See if the app is in your previous purchases in the Mac App Store, and try downloading from there.
Otherwise… See the following for one possibility: Imovie for old Mac OS - Apple Community
md71111a isn’t a valid model. The last three of that Apple part number format would probably be LLA or LL/A. There is an MD711LL/A, which is a MacBook Air 11” mid 2013 with 128 GB. That Mac can get to Big Sur (macOS 11), but 128 GB is going to be tight.
Getting Big Sur: How to get old versions of macOS - Apple Support
El Capitan is, well, ancient. It’ll have issues with all sorts of things, including with establishing secure network connections. Some of these can be fixable, such as loading a new root cert for LetsEncrypt, and some others will be more problematic.