pthread is used in the code with capital Pthread which is wrong
at least in the concurrency intro part.
We’ve added the explanation of why we are using "Pthread" in the hint widget.
Please have a look at the following lines from the lesson:
The actual call should be to lower-case pthread_create(); the upper-case version is our own wrapper that calls pthread_create() and makes sure that the return code indicates that the call succeeded. See the code for details.
If you’ve any query related to it, please let me know 
Happy learning🎉