i cannot create new folder in external hard drive

Just to add, if it is formatted as NTFS for Windows you also can't write to it. It would either have to be reformatted for Macs which would erase the drive, or you would have to install a third-party app such as Microsoft NTFS for Macs from Paragon Software.

If the external drive is being used for Time Machine you can't copy files or create a folder on it. It would be read only.