I get an error “The operation could not be completed” when trying to open files on my MacBook Air

First, try restarting the computer. Many times a simple reboot can fix odd issues.

Try running Disk Utility First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Even if the First Aid summary says everything is "Ok", click "Show Details" and scroll through the report to see if any unfixed errors are listed. If there are errors listed, then run First Aid again until they are gone, otherwise you may need to run First Aid while booted into Recovery Mode.

In case your issue is not related to file system issues, then run the third party app EtreCheck and post the complete report here so we can examine it for possible clues.

If you cannot download & run EtreCheck while booted normally, then try booting into Safe Mode to see how things work. If Safe Mode works, then it indicates you have an issue with some third party software that is installed. Try running EtreCheck while in Safe Mode...while not as good, it will be better than nothing.

"I get an error “The operation could not be completed” when trying to open files on my MacBook Air: when even i drag anything or open a file it keeps saying The operation can’t be completed An unexpected error occurred (error code -8072) my desktop started not to work."

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Transferring Issues:

Are you using an external drive here? If so, it may not be formatted properly. APFS is the format for your macOS. So, export the items to your other computer, and then format this drive on your Mac to ExFAT. Once formatted it should be readable across devices. See here: Disk Utility User Guide for Mac - Apple Support