Sophia here from Shopify. Welcome to the Shopify Community!
How long ago did you complete the first test? I wonder if your plan may have changed between then and now. If your store is in the free trial period or on a paused plan, you'll need to move to the Basic plan or higher before you can access the checkout.
What I'd recommend if you're not yet ready to switch plans is saving the checkout testing for the last step. Once you're ready to start selling, select the plan you'd like to use and just keep your store password protected while you test the checkout. Then you can remove the password protection when testing is complete. In the meantime, here are a couple resources that may come in handy for you while you're working:
Please let me know if you have any questions. If there's anything else in particular that you're working on before launching the store, I'd be happy to help.