This is a very common error that allot of users encounter.
LaraPass is build using the Laravel Framework which follows the MVC structure, meaning each URL corresponds to a particular route and not a direct file/folder.
If you have uploaded larapass_source files in your public directory, then you will be able access the installation menu by visiting
yoursite.com/public/admin/install
But this is very dangerous as you will be exposing your secure environment files on a publicly accessible domain (i.e., yoursite.com/.env).
In order to have a secure installation, you should place the source files outside the public folder (namely in the root folder) and then either create a symlink between larapass_source/public folder and your public_html folder or create a new sub-domain and have that sub domain's root be the larapass_source/public directory as shown here.
If you are still facing any difficulty and require additional help or have suggestions about how we can make this more clearly explained, feel free to open a support ticket and we would be more than happy to resolve any issue you may have.
-LaraPass Support