using an iMac as a monitor for the new MacBook Air
Can I use an older iMac as a monitor for a new MacBook Air?
MacBook
Welcome!
Only a very limited range of iMacs can be used as an external monitor, and we do not know which sub-model you have. You can look at this Apple article:
Use your iMac as a display with target display mode - Apple Support
Also, the iMac to be used as the monitor must be running no higher macOS than 10.13 High Sierra.
You can find your iMac sub-model by doing "About this Mac..."