Can I up grade my iMac 10.13.6 macOS High Sierra?

Without knowing the model year of the iMac your question is impossible to answer. If you are unsure, please click the symbol in the upper left of the display and then select About this Mac and you will find the model year there. Then check the link provided to see the last version of Mac OS you can install on your particular machine.

See macOS Monterey is compatible with these computers - Apple Support

What model and year is your iMac?

What system are you currentl running?

What system do you want to upgrade to?

The latest system your model can run is High Sierra, 10.13.6.