They can both be used. It depends on the context of the sentence and how you can use the prepositions "on", or "in". Some example sentences would be: "My mother was influential on the person that I am today.", "They were influential in the way that they spoke to me." Hope this helps.

influential in

I agree with Ben. You may also find this link interesting as it gives a range of prepositions that can follow 'influence'. I am not sure how reliable the percentages are as there is no source cited! https://lingohelp.me/preposition-after-verb/influence-by-in-on-at-from/