Freetrade aren’t here to give you tech support. They provide an investment platform. It’s on you to make sure you have enough storage available on on your phone etc. If you have a lower-end device - which typically have less storage than more expensive flagship phones - then running out of space is a real possibility.
If you lack space, it’s quite common for the Play Store to suggest that you remove apps that you’re not frequently using. It’s also very common for an application to require you have the latest version. In other words, they don’t want to expose people to bugs/want everyone on the same build for the sake of simplicity. So I wouldn’t be alarmed if the app is sending you back to the Play Store.