Plastiq payments to recipients with no prior relationship coding as a cash advance is a Visa-specific issue. The easiest way to get around this issue is to make a small payment to your intended recipient using an Amex or MC, first; (I'm not sure if you have to wait for the recipient to deposit the payment) before paying with a Visa. Your payment using an Amex or MC will establish a relationship with the recipient and you shouldn't have any further issues paying that recipient using a Visa. (If you get the cash advance warning again, just wait for the initial payment to be deposited before trying again). Keep in mind that Plastiq payments can take up to two weeks to deposit.
On the subject of Plastiq, a few months ago, Plastiq stopped processing mortgage payments using Visa cards; and a few weeks ago, Plastiq stopped allowing mortgage payments using Amex cards. This rule against mortgage payments using Visa and Amex cards has been spottily enforced and YMMV. If your mortgage lender is a big bank or has a name that obviously has to do with mortgages in it, your lender was very likely blacklisted by Plastiq and you'd get an error message if you tried to use a Visa or Amex card to pay them. If the lender is a smaller bank, credit union, or financial institution, or doesn't have "mortgage" in their name, a lot of people were reporting that they could still make mortgage payments using a Visa or Amex card through Plastiq.
So this DP couldn't make a mortgage payment using Amex through Plastiq to "Regions Bank d/b/a Regions Mortgage," so he changed the name (by creating a new "recipient") to just "Regions Bank" and it worked!
See:
https://www.reddit.com/r/churning/comments/72rj2d/daily_discussion_thread_september_27_2017/dnlk69x/ I imagine DP probably categorized the payment as "Rent/Real Estate" too. Note that if this trick also gets around the Visa-ban, making a mortgage payment using CIP via Plastiq is currently earning 3x UR.