The Raspberry Pi OS, like many other Linux distributions, is built around a philosophy of free and open-source software. Rejection of intellectual property law constraints or digital rights management (DRM) gatekeeping is part of the community ethos. Unfortunately, we live in a broader world of corporate legal nonsense. If you have subscribed to Netflix, Amazon Prime, Disney Plus, Hulu, or other major streaming services, you still need to play the game by their rules. That may require installing DRM software if it was not included by default in your web browser. Here's how to do it on a Raspberry Pi 4.
Step 1: Update your software packages
Launch the Terminal command line interface by clicking the icon or using the keyboard shortcut Ctrl+Alt+T and type
sudo apt update
Note that you will need to have the password to modify the system. In most configurations, the sudo command (Super User DO) will allow such processes without actually logging in as the root administrator account from the start.
This updates the apt (Advanced Packaging Tool) software manager which tracks software installation and versions on Debian-based systems. Updating the list tells it what needs to be upgraded.
Step 2: Upgrade your software packages
Once the update list is, well, updated, type
sudo apt upgrade
Then confirm if prompted and let it do its thing. This makes sure you have the latest version of all programs, including your web browsers. The full-upgrade command can be used instead if your release is due to upgrade to a new version. This happens about every 6 months.
Step 3: Install Widevine DRM software.
This feels unclean.
sudo apt install libwidevinecdm0
You may need to relaunch your Chromium or Firefox browser, but no system reboot should be necessary.
Step 4: DELETE (optional)
If you decide you don't want this heretical corporate nonsense anymore, there is a solution.
sudo apt-get remove libwidevinecdm0
In fact, if you want to purge it with fire,
sudo apt-get purge libwidevinecdm0
sudo and apt commands are powerful tools. The first three steps can be combined in a single command if you know what you're doing, but this is meant as a guide to newbies. Remove should keep settings, but purge wipes out everything. Be aware that removing or purging packages can screw up dependencies if other programs rely on them. This one should be safe to remove though, since it is layered on top of browser functionality instead of supporting other software itself.
If you're unsure about using these commands, that's probably healthy caution. On a Raspberry Pi, why not make a mirrored backup of your MicroSD card? They're cheap, and you have a fallback in case you brick the system. Otherwise, browse Linux and Raspberry Pi forums. Odds are, a search engine or forum thread search will find dozens of answers to any questions you may have, including, "How do I mirror my Micro SD card?"
Have fun! I recommend The Tick and The Grand Tour on Amazon Prime.
Fellow Linux people: Did I screw anything up in my efforts to simplify this explanation? Non-Linux people: Did I actually explain anything for you? Comment for a chance to earn $PIZZA!