I get an error when using File Transfer: could not find files for public key authentication

So I setup a droplet (DigitalOcean). Now I’m trying to connect Textastic using sftp but i am getting an error: could not find files for public key authentication? The info on the manual is not clear when saying For this to work, you need to transfer the private key file for your SSH account to Textastic. Example using Dropbox? What does that even mean?! Jesus!!