Hi jean-mathieufromparis,
Thanks for using the Apple Support Communities!
I understand that you haven’t been able to scan from your Canon MG5550 printer since updating to El Capitan. Your printer does appear to be supported for both printing and scanning according to the following Apple support document.
Printer and scanner software available for download for your Mac - Apple Support
To begin troubleshooting, I would first suggest checking for software updates as suggested by the article to ensure you have the latest software on your Mac.
Check for software updates before connecting your printer or scanner. This updates the list of the latest supported printer and scanner models on your Mac. If you don't check for updates before trying to connect a new printer, you might not see available software when you connect your device. If you're using a Wi-Fi printer, be sure to connect your printer to your Wi-Fi network before adding it to your Mac.
If you continue to have issues scanning, read over and work through the following articles as well.
OS X El Capitan: Scan images or documents
OS X El Capitan: Scanning troubleshooting
Best!