Every content in the Stremio is provided by Add-ons, this applies to movies, series and subtitles.
No streams available
If you see information that no streams are available for particular movie or series you can try install more Add-ons (or install any of them if you haven't yet). Here you can find instruction how to install Add-ons https://stremio.zendesk.com/hc/en-us/articles/360021348391-How-to-install-uninstall-Add-ons
No subtitles available
Subtitles are provided by OpenSubtitles Add-on. Opensubtitles is webpage where community users add subtitles created by them to the open subtitles database. If there are no subtitles available for particular movie or series there is nothing Stremio can do about it. You can try to search for the subtitles on the other webpage and simply Drag&Drop them to the stremio.