I want to update my MacBook to IOS 10.15, but it is not listed as an option

Should your MacBook (?) be unable to upgrade to full Catalina 10.15.7

could be the model may have issues; perhaps that is not compatible.

• macOS Catalina is compatible with these computers - Apple Support

https://support.apple.com/en-us/HT210222

Since there had been many different portable Macs with 'MacBook name'

there always remains a possibility ~ one might not use Catalina's installer.

• How to get old versions of macOS - Apple Support

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

Good luck & happy trails!🐌🌿🤖🌻🐢

iOS is for iPhones and iPads.