Not enough storage space on MacBook Air SSD
You could just drag the files from the iCloud folder to the DropBox folder.

Do them a bit at a time to ensure that you have enough disk space to complete the transfer.

If your available disk space is less than the size of the largest file on your iCloud Drive (i.e. you have few, very large files) - you could upload it on the DropBox web site to store it in Dropbox without requiring extra local storage right away.