No, that would be "non posso lasciarvi per fare questo"; "to leave" can't be modal, so you need to use "to let" as a translation.
Can someone help me understand why we use the past imperfect here when we are talking about something that is about to happen?
There is no imperfect in the Italian sentence; if you mean "lasciarvi", that's the infinitive lasciare with the clitic vi attached.
Grammar is not my strong point lol. Thanks, actually your answer helps me a lot, I can understand it now.
The problem is, instead of learning simple infinitives here, we are learning infinitives with clitics attached to them. This is beyond frustrating! Do we use "vi lasciare" or "lasciarvi"? There is no explanation at the beginning of the unit to tell you what expectations are being made by the program checking your answers.
You can only separate the clitic from the infinitive when it's introduced by a modal verb as in this example: "non posso lasciarvi fare questo" and "non vi posso lasciar fare questo" are both correct (the clitic must precede the modal or auxiliary verb).