Sololearn doesn't ask you money for the courses. If you want to purchase the Pro pack then only you have to pay.
I have often seen that whenever I try to proceed from one host to another it also shows that its locked. Probably that's a bug.
SoloLearn's slogan is "Learn To Code for Free".
You don't need to pay for learning.
If your 7 day free trial of PRO subscription expires, you can email to [email protected] for cancelling the PRO subscription.
Otherwise, if you want to support SoloLearn, add credit card details to Google account and pay with Play Store.
Sololearn is a free learning app. It shows the lock material because you haven't bought the PRO subscription package. You can unlock the material by sequentially solving all the materials before it . If you want to unlock a particular material without solving previous material you need to buy a subscription.
It's probably a bug or you may be trying to study the next concept without going in the order app provided,
Just follow the order of concepts given in order, if you already know then then directly do the questions and then you may proceed..