python-qgis was not found while installing QGIS on Linux Mint

I am trying to install QGIS on my Linux Mint 20.1 Cinnamon, using this:
https://freegistutorial.com/how-to-install-qgis-on-linux-mint-19/

But when trying to

sudo apt-get install qgis python-qgis qgis-plugin-grass

it says python-qgis wasn't found.
I tried sudo apt-get install qgis python3-qgis qgis-plugin-grass
it showed it depends on following packets :
libgdal20 libqgis-3d3.16.3 libqgis-analysis3.16.3 libqgis-app3.16.3 libqgis-core3.16.3 libqgis-gui3.16.3 python3-qgis qgis-providers qgis-plugin-grass grass740 libgdal20 libqgis-app3.16.3 libqgis-core3.16.3 libqgis-gui3.16.3 libqgisgrass7-3.16.3 .
I tried install them all together with apt-get install it showed
libgdal20 , grass740 ,libgdal20 (no installation candidate exits ) what is that now , I deleted those 3 packets, it showed me another dependecy problems.
here is for sudo apt install qgis qgis-plugin-grass :

You have requested an impossible situation or when you have the
Unstable distro use that some required packages still
have not been created or have not yet left Incoming.
The following information may help you resolve the situation:

The following packages have unfulfilled dependencies:
 qgis: Depends on: libgdal20 (> = 1.8.0) but cannot be installed
        Depends on: libqgis-3d3.16.3 should not be installed
        Depends on: libqgis-analysis3.16.3 should not be installed
        Depends on: libqgis-app3.16.3 should not be installed
        Depends on: libqgis-core3.16.3 should not be installed
        Depends on: libqgis-gui3.16.3 should not be installed
        Depends on: python3-qgis (= 1: 3.16.3 + 28bionic) but should not be installed
        Depends on: qgis-providers (= 1: 3.16.3 + 28bionic) but should not be installed
 qgis-plugin-grass: Depends on: qgis-provider-grass (= 1: 3.16.3 + 28bionic) should not be installed
                     Depends on: but grass740 cannot be installed
                     Depends on: libgdal20 (> = 1.8.0) but cannot be installed
                     Depends on: libqgis-app3.16.3 should not be installed
                     Depends on: libqgis-core3.16.3 should not be installed
                     Depends on: libqgis-gui3.16.3 should not be installed
                     Depends on: libqgisgrass7-3.16.3 should not be installed
E: Problems cannot be corrected, you have defective packages 

when even try install these dependecies they count on others