would 'être capable de voir ce film est important' also be acceptable?
In this case, yes, because the English version does not mean that it is important... to me.
I just wrote 'que puisse voir...' and got narked wrong for missing 'je'. Do i have to put it or 'on', or am I already right?
If you want to use on: qu'on puisse / que l'on puisse (more formal) voir...
How about "C'est important que je puisse voir ce film"?
For sure, that is correct. However, Duo wants to stress how the English gerund translates to French (subjunctive).
Note that you could also use an infinitive: "Pouvoir voir ce film est important pour moi".