Why can't I tick another payment gateway on my Payment Gateway page?

Your Online Payments plan sets how many gateways you can run at once. Go to Settings → Store Settings → Payment Gateway and look at the Payment Gateways box: it shows how many you have selected out of your allowance. To swap, untick one gateway, tick the new one and click Save gateways. If you need more at the same time, buy extra in the Gateway Capacity card on the same page or move to a larger plan.