Correct spelling, explanation: the verb sustain comes from Latin sustinere, which means hold, bear. There is no consonant b in this word, nor pronunciation pointing to b existing in the word, so substain is an incorrect form. Sustainis the only correct spelling.

Definition of sustain:
verb, to hold up
We need a new campaign to sustain profitability.
Less water should be wasted to sustain our planet.

Incorrect spelling, explanation: this form is a misspelling, maybe done due to prefix sub- which exists in English. However here the correct form is sustain, not substain. Sustain comes from Latin sustinere, so you can see that there was no b in the original form either.

Incorrect spelling

Incorrect spelling