How do I extend my dispay to a second computer?

No computer in the MacBook family ever supported being used as an external monitor for another computer. That was limited to a small sub-set of iMacs and is now a discontinued feature.

You can use network screen sharing but it is usually slow and you cannot do and extended desktop, only mirrored. You are far better off with a proper external monitor. It uses less energy, has faster response, and allows extended desktops.

Share the screen of another Mac - Apple Support

Target Display Mode only work in certain ways and with certain Macs.

Check here Use your iMac as a display with target display mode - Apple Support (CA) if your Macs are in. But you're trying to do it from and MBP to an MBA, and that's not possible.

There are third party options thou. Here's an exemple https://astropad.com/target-display-mode-mac-to-mac/. Never used it, so, just a possibility, but I don't know how it works.

Future macOS Monterey will have a new option for this, but still in beta, so it can't be discussed here at the moment.