If your public/private key pair was generated via PuTTYgen. BackupSheep doesn't work with PuTTY-format private keys, so you would need to convert it to OpenSSH format. To do that, please perform the following steps:
Click File -> Load private key
Go to Conversions -> Export OpenSSH and export your private key
Try to paste the converted private key to the BackupSheep website or database Integration box.