When I was set GCP APP engine setting for ssl certificate, "Add a new SSL certificate" uploading my own cert(got it from lets encrypt through certbot with debian os) I got error: The private key you've selected does not appear to be valid.
get openssl and copy your certificate pem file to the bin folder of openssl,
openssl for windows: https://slproweb.com/products/Win32OpenSSL.html
then ren openssl command line (replace “private.key” or “Privkey.pem” with your own
file name in the following command line, then you can file the privte_rsa.key file in the bin folder, then open it by text editor, copy and paste it to your Google cloud platform.
openssl rsa -in private.key -out private_rsa.key