How do I get to the web UI in CraftCMS to install it? I initially tried this with Ubuntu Linux and a .tar.gz file without the composer tool.  I was using MySQL and PHP 7.4.  My problem is about the same with my second attempt.  I will now describe my second attempt and ask for help with it (instead of my first attempt).
I am trying to install CraftCMS on Debian 10 with composer.  Everything seems to work on the back-end.  I am using PostgreSQL and PHP 7.3.
I finally went to the web front-end portion of the directions.  In a web browser I go to /index.php?p=admin/install
It resolves to /dashboard.  The web page that is rendered says &quot;Not Found   The Requested URL was not found on this server&quot;
When I just go to &quot;&quot; in the web browser, I see &quot;Welcome  Thanks for installing Craft CMS!&quot;  I can click on the &quot;Control Panel&quot;, but that takes me to a page that is Not Found.
I expect to see the screen to install CraftCMS in a web browser when I go to /index.php?p=admin/install
What am I doing wrong?
How exactly are you running Craft?
Ubuntu/Debian are just Linux operating systems, they don't necessarily come with a web server stack installed but it looks like what you're running is at least partially working.

If you getting the welcome screen, it sounds like Craft got installed so I wouldn't try reinstalling unless you can confirm there's no tables for Craft in your database.

If you can get to http://examplesite.com/index.php?p=admin/dashboard, that typically means the web server's rewrites aren't working and what you have now is a webserver configuration issue—not uncommon.


To solve a web server issue:

Apache: make sure .htaccess is located inside Craft's web folder and AllowOveride All is in your virtual host config.
Nginx: you need some special rewrite rules to get it going.
You may also need to set omitScriptNameInUrls to true in config/general.php.

If you can't get to the dashboard or the login screen, something else happened, perhaps a botched install and you should revisit how your web server got installed.