this error only appears when system is not able to find node_modules in correct place .
I guess you might have ran those commands somewhere else not in the ionic project directory . Generally its a common mistake not to naviagate into correct directory to run the commands .
Please make sure that you navigated to your ionic project directory (where your src folder is ) . You have to navigate using "cd" command in terminal or cmd in windows .
then run all the commands to install node packages .
When you run those commands a folder should be create names as "node_modules" in which packages are installed .