i dont know why my imac cant update to macos 11.13.4 when it 24-inch early 2009

Sometimes most resources are there but not all the ones required to run all features of the operating system. Since a 2009 Mac is 4 years out of support and the pattern has been once a Mac reaches 8 years old any kind of software support completely disappears then it doesn't surprise me you aren't seeing it for a 09 Mac.

Vintage and obsolete products - https://support.apple.com/HT201624

You need an iMac late 2009 or newer.

macOS High Sierra - Technical Specifications