You just need to use a valid private key so that the node can decode. That’s why you get the error.
As Jon put it in his answer (I'm having trouble signing a SHA1 string encoded in Base64 - #4 by Jon), typically the Private Key would be something like:
-----BEGIN RSA PRIVATE KEY-----
MIICWwIBAAK…
abc…
-----END RSA PRIVATE KEY-----
The Private Key must be in the correct format so the Crypto node can use it.