I have a slack app with a sign in with slack button. Earlier I was testing it on localhost with the redirect url as localhost. Is there anyway I can test the same on an internal network, because right now every time I try to do that as soon as I authorise it redirects me to the localhost.(which it should since redirect url is localhost).
I tried to use the url of my internal network as the redirect url but it does not accept it.
You can only redirect to an URL that is accessible from the public Internet. So you can't redirect to localhost or to a machine that is behind a company firewall and not reachable from the Internet.
To test your "Sign-in with Slack" button one good approach is to use a tool like ngrok that gives your a public URL and redirects traffic automatically to your local machine via VPN.
Also check this tutorial from Slack on how use ngrok.