How to view its results by going to http://localhost/osm/slippymap.html
Results of what?
To access a URL on your own machine, you will need a web server running on that machine.
We’re going to need more information to help you out here, James
I’m guessing that you were following some instructions, perhaps https://wiki.openstreetmap.org/wiki/DE:Ubuntu_tile_server or https://wiki.openstreetmap.org/wiki/Switch2osm/Translation/fr/serving-tiles/building-a-tile-server-from-packages/ , and it didn’t work.
As has already been said, without knowing what instructions you were following or what went wrong it’s difficult to comment.
Unfortunately things change - both the software that you’re trying to install and the OS you’re likely to be installing on, so instructions do get out of date. If you follow https://switch2osm.org/manually-building-a-tile-server-16-04-2-lts/ to the letter I suspect that it should still work, although in the month or so since I last updated it someone’s mentioned that it probably won’t work on Ubuntu 17 without some changes, and changes in the node.js world last week mean that some other changes could probably do to be made (though it should work as is).