Hi,
The Configure a Resource Lock exercise requires you to use your own Azure subscription. This is what they meant by "This exercise is a Bring your own subscription exercise...". You cannot use the sandbox subscription since you aren't granted permission to add/edit Locks.
If you don't have a paid or Azure free subscription to use that is okay. It is enough for you to read through the exercises and understand the difference between Delete and Read Only locks. Pay attention to the steps they asked you to do and look at the screenshots that show the error message that results when you attempt certain operations with a lock in place.
If the above is helpful please click Accept Answer.
Thanks.
-TP