Varför kan jag inte använda iphone dubblering med min macbook air m4?

If you are talking about iPhone Mirroring, see these instructions. Make sure you have updated to 18.6 and 15.6, because Mirroring may not work right in earlier versions.

iPhone Mirroring: Use your iPhone from your Mac - Apple Support

Hi,

Please ask us about your issue in English since Swedish language isn't supported by translation utility.

PS. From the system requirements for mirroring: “To use iPhone Mirroring, it must be available in your country or region. iPhone Mirroring is not currently available in the European Union.”