okular snap package doesn't appear in applications and command 'okular' is not recognized

You should be able to launch Okular by the following command

snap run okular

If you want to add it to the applications list, you may create a .desktop file in ~/.local/share/applications/ (say okular-snap.desktop) with the following content and save it

[Desktop Entry]
Comment=Okular document reader
Terminal=false
Name=Okular
Exec=/snap/bin/okular
Type=Application
Icon=okular
NoDisplay=false

You may find an already existing .desktop launcher for the application in the /var/lib/snapd/desktop/applications/ directory. In that case just copy it to ~/.local/share/applications/.

If the correct icon is not appearing:
You may set any image (preferably PNG or SVG) as the icon for the application. To do that change the Icon= line and point towards your preferred image. For example if you want use the MyIcon.png image located in your Desktop folder, change the line to

Icon=/home/YOUR_USER_NAME/Desktop/MyIcon.png

(Replace YOUR_USER_NAME by you actual username) .