In Photos, If I can adjust parallax on my iPhone 11, why can't I adjust parallax on my iMac running OS 11.1?

If you are asking about the controls to correct the horizontal or vertical perspective distortion, it simply not yet possible on a Mac. The iPhone Software is usually ahead of the Mac Software with new photo editing tools. I hope it will come to the Mac soon as well. Photos 6 on Big Sur can finally apply adjustments to videos, which the iPhone could do all along.

Or are you asking about adjusting the depth control slider on Portrait Mode Photos? There may be several reasons:

• It will only be possible, if the Portrait Mode Photo has been taken with an iPhone or iPad that supports the depth control (Edit Portrait mode photos on iPhone - Apple Support)
• It may not be possible to adjust the depth, if the photo has been transferred without the depth map. How are you transferring the photos? You will have the least problems, if you transfer them with iCloud Photos.
• If you are using iCloud Photos, the Portrait mode photo should not have been optimised to save storage. You have to wait, until the depth map will have been downloaded.
• If the depth effect has already been edited on the iPhone, you may need to revert it to the original, before e you can change the Portrait Lighting and depth.

Hello and welcome to Apple Support Communities, Bart!

We understand you want to adjust the parallax effect on your iMac. We'd be happy to provide information to help you accomplish this.

Try adjusting System Preferences according to this article: Reduce screen motion on Mac

If this is not the feature you're referring to, please let us know more so we may help you out.

Have a great day!