First check that you added the Ledger udev device descriptors as mentioned in the Linux installation guide. Next, check that your Ledger wallet is recognized by the Oasis Core Ledger tool. You should see something like this:

$ oasis-core-ledger show_address
oasis1qp8d9kuduq0zutuatjsgltpugxvl38cuaq3gzkmn
Ensure account address shown on device's screen matches the outputted address.

If all of the above works, then the issue is most likely that Chromium does not have the permission to access your Ledger device. Starting with Ubuntu 20.04 the Chromium browser is installed via snap package by default. Snap is more convenient for upstream developers to deploy their software and it also adds additional layer of security by using apparmor. In our case however, it prevents Chromium to access arbitrary USB devices with WebUSB API including your Ledger device. A workaround for this issue is to install Chromium natively using the official Chormium beta PPA or the official Google Chrome .deb package.