Linux MInt has taken the decision to disable the ability to install snapd and subsequently containerised apps (or snaps) from the Ubuntu Snap Store.
You can disable the block on installing snapd as follows.
sudo rm /etc/apt/preferences.d/nosnap.pref sudo apt update sudo apt install snapd
This will allow you to install those specific containerised versions of apps that you want, e.g. Whatsdesk (a containerised Whatsapp client for your Linux desktop).
sudo snap install whatsdesk