The operation can’t be completed because you don’t have the necessary permission. Mac desktop.

If you look for the information of this Folder or File and the navigate to the down right of this window. There should be an option to edit the permissions of the users. Then try to change the permission of your username to "Read and Write".

If it's a System operation you try to complete, it would help you to login as root. This user has all permissions.

How to enable the root user on your Mac or change your root password - Apple Support

It would also help to know which operation you try to compete.

Cheers

Does this only happen when you are trying to download things?

In the Finder, press command-shift-H to open your home folder, locate Downloads in there, select it and press Command-I. Post a screenshot.

If you are using Safari, you can temporarily try selecting a different location for your downloads, by going to Safari->Preferences->General and selecting a different folder. Of course this should be sorted out, but in the meantime it may allow you to download Etrecheck so we can proceed with troubleshooting.

There is no reason to login as root, and in fact the root account is disabled by default.

There appears to be some problem with your account, but we need more specific information.

We have no idea yet what your mac is, what version of the OS, etc.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

You have to give sufficient information so we can help.

What are you trying to do when this happens? What operation is this? In what application?