Authentication - Cannot Choose Private Key File

You're using grossly outdated software from 2012.  Frankly, that doesn't bode well for you.

Meanwhile, security threats have necessitated higher standards in server connection protocols.  If you don't upgrade to modern web authoring software (highly recommended), at the very least, use a more secure S/FTP client like free FileZilla, Cyberduck or Transmit by Panic to connect with your server.

Adobe Dreamweaver CC - https://www.adobe.com/products/dreamweaver.html  (free 7 day trial available).

Screenshots from Dreamweaver CC -

Connection options.

If private Key file is selected, use OpenSSH .pem key file.

OTHER CODE EDITORS:
-- Codespaces (free, browser-based) - https://github.com/features/codespaces
-- Nova (Mac only, formerly called Coda) - https://nova.app/
-- Pinegrow - https://pinegrow.com/
-- Sublime Text - http://www.sublimetext.com/
-- Visual Studio Code (free) - https://code.visualstudio.com/
-- Wappler ~ Visual Web App Builder - https://wappler.io/

Hope that helps.