Try running command 'whereis nginx'. It will give you the correct path of the nginx installation, in my case nginx is installed in '/usr/local/sbin', so I need to check if this path exists in output of command 'echo $PATH'. If you don't find the path in the output of this command, then you can add this.
Suppose the output of my 'echo $PATH' command is this:
~$ echo $PATH
/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/nginx/sbin
Then I can append the path '/usr/local/sbin' in $PATH by following command:
~$ echo 'export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/nginx/sbin"' >> $HOME/.bashrc
Please check your nginx installation path may differ from mine, but the steps for adding them are same.