Trying to set up SMTP mail Gmail

Question

I am unable to set up Post SMTP mailer. When I get the key and secret and then attempt to grant permission with google, I get this error:

redirect_uri_mismatch

I am signed into the Google suite on the email address I am attempting to set this up. I have tried searching for instructions on how to correct it and found nothing. One guide I followed told me to sign into Gmail and go to advanced settings, but I cannot find where that is located. I have searched through all the answers on here but I don't know what to look for. I did have functioning email and now I don't and I don't know why. Please keep answers simple, I don't understand computer jargon.