Hello George,
Good morning. I successfully changed the format and was able to connect 
If anyone else has the same issue I used the following command ‘puttygen private_key.ppk -O private-openssh -o private_key’
One issue I did find was that my private keys have different passwords to my sFTP passwords. When connecting in Wappler, you only have the option of entering one password for the sFTP account. The only way to negate this is to either have no password for your key or have the same password for key and sFTP account. I chose the latter and was connected.
All the best,
Brian