At every one I have been to (not a lot I admit, maybe 4 in my life), I had to both provide insurance card AND pay my copay before a doctor would see me. Without payment, they send you to go to the ER.
How do you know they would send you to the ER? Did you not have insurance? Did they actually send you to the ER for not having it? Honestly, sounds like you simply had insurance, filled out a form and just assumed it was required. I've been to multiple walk-ins, and you can visit without insurance. They will ask for insurance up front, but you can simply say you don't have any. Then you will offer payment options. The average Urgent Care visit is under $200. The point of Urgent Care is literally to handle things that don't need ER visits, and at a much lower rate. I don't know why I've been down voted above, you can simply Google/Bing/Duck "no insurance Urgent Care" and find tons. The CVS MinuteClinic, which is everywhere, doesn't require insurance for instance.
https://www.cvs.com/minuteclinic/insurance-and-billing