Cannot create cache directory or directory is not writable. Proceeding without cache

Change the group permission for the folder

sudo chown -R User /home/User/.composer/cache/repo/https---packagist.org

and the Files folder too

sudo chown -R User /home/User/.composer/cache/files/

I'm assuming User is your username, if not change the 4th parameter to your username.

If the problem still persists try

sudo chown -R User /home/User/.composer

Now, there is a chance that you won't be able to create your app directory, if that happens do the same for your html folder or the folder you are trying to create yourproject in.

Hope this helps.