It is necessary to make it clear where you want to create the folder, be it the folder on your Mac internal hard drive, or an external hard drive. It may relate to permission issues if you can't create a file/folder on a Mac. Also, system glitches could be the reason.
You can create a file on Mac via Finder, the built-in file management tool via its menu bar. Make sure you have selected the desired folder where you want to add a new file/folder to it from the sidebar. You can create a new folder under Recents and some system folders. Thus, you may find the New Folder option greyed out.
In addition, if you can't write to the internal hard drive or the external hard drive, it could be caused by permission issues. As you have logged in as admin, you should have full read and write access to the Macintosh HD. For an external drive, make sure the drive is writable. You can right-click the drive icon and click Get Info, expand Share & Permission to change hard drive permissions on Mac.
Perhaps the read-only external hard drive is formatted with NTFS, a file format for Windows platform. You can choose to format the external hard drive on Mac to make it compatible but this erases all contents on the drive. Or, you can try professional NTFS drive for Mac such as iBoysoft NTFS for Mac to read-write the drive without limitation.
Besides, the issue that you can't create new folder on Mac occurred after upgrading to macOS Big Sur, the system glitches can not be ruled out. You can try to reinstall macOS, downgrade macOS, or update macOS to the latest version to see where it works to fix the issue.
Eudora Liu
Vain Rowe
Jenny Zeng
