Magento 2.3 is under Beta testing at this point. Any errors that you get from this version needs to be reported like said on their page :
I understand that this can be frustrating but i am afraid that you will not receive a lot of answers here regarding the PWA or other new features of Magento 2.3 since it is not ment to develop or run a website on. It is a testing environment.
Other then that it can be that your permission are not set correctly on the pub folders. If this is a testing enviroment you can test that by setting the permissions to 777 of the pub directory. If it is not a testing enviroment be sure it is set to 775. Also if you are using Apache or Nginx and you use Virtualhosts be sure the user has permissions to access that directory.
Not sure which operation system you use to provide you with the right commands.