You can only claim expenses for which you have not been reimbursed. So if your client gave you $500 for vehicle expenses, but you spent $600 for fuel, you can only claim the $100 difference. You’ll need to have receipts & documentation (like a logbook), just in case the CRA requests it.
If you have TurboTax Live Assist & Review, here is how to talk to a Tax Expert: https://turbotax.community.intuit.ca/community/navigating-turbotax/help/i-m-using-turbotax-live-assist-review-how-do-i-talk-to-a-tax-expert/00/873764