Is it possible to make changes to my tax return if it has already been submitted? I did not add my car payments

Your pending return can not be changed. If your return is accepted you will get an email. If the return is rejected the e-mail will tell you what to correct and you can make changes before e-filing again. Amend to change an accepted return after you get your refund. Instructions below

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...

Not sure why you want to add car payments.  You are posting from online Deluxe, which does not indicate that you were self-employed.   You cannot add vehicle expenses unless you are self-employed and are using a Schedule C.

But....if you e-filed...you cannot stop it or change your return now.  Once you e-file, your return is in "pending"---where it will sit on a server for a few weeks until the IRS begins to accept/reject returns.  You cannot make any changes while your return is in pending.  If the IRS accepts your return, you have to wait until it has been fully processed in order to make changes by filing an amended tax return on a special form called a 1040X.   If they reject your e-file, then you can make changes and re-file.