No, it is not a scam.
After you reach the $20 threshold, it is best to wait a couple days. If you try to withdraw it immediately, they think you might be doing a "cut and run". Apparently there are people who do this.
I would open a request with Ibotta Care and let them know you are having a problem withdrawing. Please note it can take a couple of days to get a reply.
After you have used the app for awhile, there should not be any issues with withdrawing.
