"Expecting: ANY PRIVATE KEY"

[Mariano Gedisman-Córdoba]

Hello everyone!
I have googled this error to no avail, but all answers don’t seem to adjust to my particular case, or maybe I’m too novice at figuring it out.

I have this key file:

-----BEGIN OPENSSH PRIVATE KEY——
[key here]
-----END OPENSSH PRIVATE KEY——

I’m trying to use this key in order to log in to my Google cloud instance through browser console, and I get the following error:
"Error: Failed to read key. The key file must be ECDSA or RSA in PEM format. "

I googled how to achieve this, and tried the following on my local machine:
$ openssl rsa -in id_rsa.txt -out id_rsa.pem -outform PEM

Sadly, I run into this error:
unable to load Private Key
56081:error:0906D06C:PEM routines:PEM_read_bio:no start line:/BuildRoot/Library/Caches/com.apple.xbs/Sources/OpenSSL098/OpenSSL098-59.60.2/src/crypto/pem/pem_lib.c:648:Expecting: ANY PRIVATE KEY

As I said, Google did throw some clues, but nothing conclusive.
Note: I am running OS X El Cap.

Can anyone point out what am I doing wrong?

Thanks a lot in advance and I wish you a really good week!