Hi,
Unfortunately, not all training modules are set up to have you use a no-charge sandbox to complete an exercise. Instead they will typically let you know that you can sign up for a free Azure account and use that. The free account gives you $200 Azure credit to spend within 30 days, then after that you can continue to use the free tier services for 11 months (total of 12 months). After the 12 months there are several free tier services you can continue to use since they are not time limited. This does require a credit card, and after the first month you need to be careful not to use an option that isn't free.
If you are not ready to create a free Azure account right now, you can get around it with some creativity. Many other learning paths provide sandboxes, and you should be able to use one of those to complete exercises that don't have a sandbox. To make it more convenient you could open a separate tab with sandbox. You can use portal in sandbox as well, not just cli.
Make sense?
-TP