I have a MacBook 12 retina year 2017 I would like to upgrade my macOS to Catalina but I can't do it

Downloadable full installers for Upgrade versions of macOS are here:

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

"Safari uses these links to find the old installers in the App Store."

After downloading from the App Store, the installer opens automatically.

• macOS Catalina 10.15 can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Mojave 10.14 can upgrade High Sierra, Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion
• macOS High Sierra 10.13 can upgrade Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion

check compatibility

• macOS Catalina 10.15 hardware requirements
• macOS Mojave 10.14 hardware requirements
• macOS High Sierra 10.13 hardware requirements

Good luck & happy trails!

Follow the advise of @K. Shaffer and the should work fine.