Correct permissions for Writable folder?

Could somebody show me what permissions do I need to setup for Writable and it's sub folders?

I am getting this error

CodeIgniter\Cache\Exceptions\CacheException

Cache unable to write to /var/www/myproject/writable/cache/

I am on Ubuntu 18.04 using CI4 beta 1.

writable folder and child folders must be owned by web server process, in my Linux Centos is apache (production)
    chown -R apache /var/www/myproject/writable

Or allowing total control to everyone (development)
    chmod 777 -R /var/www/myproject/writable

As you are on Ubuntu, if you have root access you can set
 chown -R www-data:www-data var/www/myproject/writable