It’s not clear from your post, but I am guessing you might be trying to copy code from the webpage which got copyrights applied and they blocked copy/paste function.
To test it, try to open any other webpage ie theAge.com.au and try to copy/paste using same key combination (Command+C to copy, then Command+V to paste on MAC).
If the combination does not work for you in any program, check the settings of the shortcuts on your device’s keys. In the meantime, it should work to select text and drag without releasing the mouse
The first response did say to use control and not command, so maybe that’s where you got confused. Lol. (Windows users generally have no idea about Mac idiosyncrasies.)