When I tried to download the iMovie app it said that i needed a software update, but when I go to updates, there are non updates available

The current version of iMovie, sold at the App Store, is iMovie 10.14.1. The compatibility listed requires macOS 10.14.6 or later.

Previously, it has been possible to install an older version of iMovie, where the system requirement is macOS 10.13.6.

Your installed system version is rather old and does not suffice to get any currently available version of iMovie installed.

You do not need a software update, you need an upgrade. The upgrades are not shown in the updates pane of the AppStore.

• Try to upgrade to Catalina, if your Mac supports it. The requirements are listed here: How to upgrade to macOS Catalina - Apple Support. Catalina will make 32-bit applications incompatible. In that case check the other options below. learn about 32-bit app compatibility
• Or try, if upgrading to High Sierra will be possible, if your Mac is too old for Catalina: How to upgrade to macOS High Sierra – Apple Support
• You could also try to upgrade to Mojave, if you have many 32-bit applications installed, that will no longer work on Catalina: Mojave