Android has "Internal Storage" (the device itself) and "external storage" (the removable SD card in the device).
Is the 20gb free on the Internal or External storage?
Android installs APKs/apps ONLY on Internal Storage. If there's not sufficient capacity on the device itself, it's possible you may need to transfer some images or files from internal device storage to the SD card external storage - or uninstall apps you never use to make space.
Under settings, there's Apps > Special app access > Install unknown apps. As well as a possible Trash folder that is full of things you deleted which are simply sitting in the Trash folder - located under settings > Storage > Trash. You can tap that and empty the trash, freeing up storage.
If the previous version of LTA is still installed, you need to uninstall it (after you move your save files to a secondary backup location) before trying to install LTA again. Android doesn't manually overwrite any APK.
Save files should be found here in a HIDDEN folder you need to UNHIDE: SD:/Android/data/lta.com/files/save
Be sure to put them back once the new version of LTA has been installed.