I suggest you please use a new Developer Org / Trailhead Playground for validating the challenge. If you are using a brand new developer org then please double check that your org doesn't have a namespace enabled.
If the problem persists, log out of all accounts and log in once again in a different browser by clearing the cache and cookies which should probably do the trick.
I hope it helps you.
Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in future.
I trust you are doing very well.
I suggest you please use a new Developer Org / Trailhead Playground for validating the challenge. If you are using a brand new developer org then please double check that your org doesn't have a namespace enabled.
If the problem persists, log out of all accounts and log in once again in a different browser by clearing the cache and cookies which should probably do the trick.
I hope it helps you.
Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in future.
Thanks and Regards,
Khan Anas