Which download did you mean?
An APK installs an Android application. A movie download stores video for later playback. My List keeps a title you want to revisit. These three tasks need different controls, so start with the one you actually want.
Before a flight or a long journey
Choose your movie while connected, then check whether its viewing application offers offline access on that device. Finish the download and test it without a network before leaving. Include the desired audio and subtitle tracks in that check.
When a download says unavailable
A disabled button may apply to that title, device, account or app version. The OnStream catalogue cannot restart another application’s download service. Use that application’s own help rather than replacing working browser settings.
My List does not store video
A saved poster remains a reference to the film or programme. It is not evidence that the entire video is stored on your phone. This site does not supply standalone film files or an offline streaming cache.