Can't unlock pgpy private key: NotImplementedError: <SymmetricKeyAlgorithm.Plaintext: 0>

Question

I am trying to load a pgpy private key and decrypt an encrypted text message.

Private_key_path = r"/content/Private_KEY.sub"
privkey, _ = pgpy.PGPKey.from_file(Private_key_path)

The key and the encrypted message manage to load without problems:

file_path=r"/content/we.csv"
message_from_file = pgpy.PGPMessage.from_file(file_path)
print(message_from_file)

The encrypoted message shows correctly:

-----BEGIN PGP MESSAGE-----
Version: BouncyCastle.NET Cryptography (net6.0) v2.0.0+4f800b4d32

hQEMA7yVCvCXVxN9AQf/Wegn6Dxk/zLWn594RJ5QAgCtmU0F+Yh3P4moL8UKuTLc
eifxnuG88dtpUpOuzc5cu9w84EBnQq+l8fMszuy0dMB6wkvbNtRZ03bOzJv1vkAD
4tudbbEH1+YfGqYj2gJRZ9LAprH/KtqL52SzUBmXdG9NrUnjFhIT3sWw6b+tfvMR
pZgpg6O1PsyIw1xdvCjoRjLyNT1eyvNw1nUP1wEi9G2blFlvsxAJnUo/SxD2qTVr
...
/dAc+aGk+DU0cHeA+P/Gon9Io2jPpgt3Ur9uahQ3mRvgpLBgvDsxD1ZhXZd44Dj4
4h+p30SJoeQUYh9lD7wsrQl9wUspo4p+jULSQRmps4wDv4KKLk/pt+ZBEQhhnJmR
/O9d2ZfL31BY1GV9bg==
=hw2Z
-----END PGP MESSAGE-----

When I try to unlock the key with the passphrase though:

with privkey.unlock('correctpassphrase') as key:
  raw_message = key.decrypt(message_from_file).message
  print(raw_message)

I get the following error:

NotImplementedError                       Traceback (most recent call last)
<ipython-input-139-f188768ef141> in <module>
----> 1 with privkey.unlock(PASSPHRASE) as key:
      2   raw_message = key.decrypt(message_from_file).message
      3   print(raw_message)

6 frames
/usr/lib/python3.8/contextlib.py in __enter__(self)
    111         del self.args, self.kwds, self.func
    112         try:
--> 113             return next(self.gen)
    114         except StopIteration:
    115             raise RuntimeError("generator didn't yield") from None

/usr/local/lib/python3.8/dist-packages/pgpy/pgp.py in unlock(self, passphrase)
   1809         try:
   1810             for sk in itertools.chain([self], self.subkeys.values()):
-> 1811                 sk._key.unprotect(passphrase)
   1812             del passphrase
   1813             yield self

/usr/local/lib/python3.8/dist-packages/pgpy/packet/packets.py in unprotect(self, passphrase)
    939 
    940     def unprotect(self, passphrase):
--> 941         self.keymaterial.decrypt_keyblob(passphrase)
    942         del passphrase
    943 

/usr/local/lib/python3.8/dist-packages/pgpy/packet/fields.py in decrypt_keyblob(self, passphrase)
   1351 
   1352     def decrypt_keyblob(self, passphrase):
-> 1353         kb = super(RSAPriv, self).decrypt_keyblob(passphrase)
   1354         del passphrase
   1355 

/usr/local/lib/python3.8/dist-packages/pgpy/packet/fields.py in decrypt_keyblob(self, passphrase)
   1252 
   1253         # derive the session key from our passphrase, and then unreference passphrase
-> 1254         sessionkey = self.s2k.derive_key(passphrase)
   1255         del passphrase
   1256 

/usr/local/lib/python3.8/dist-packages/pgpy/packet/fields.py in derive_key(self, passphrase)
   1017     def derive_key(self, passphrase):
   1018         ##TODO: raise an exception if self.usage is not 254 or 255
-> 1019         keylen = self.encalg.key_size
   1020         hashlen = self.halg.digest_size * 8
   1021 

/usr/local/lib/python3.8/dist-packages/pgpy/constants.py in key_size(self)
    237             return ks[self]
    238 
--> 239         raise NotImplementedError(repr(self))
    240 
    241     def gen_iv(self):

NotImplementedError: <SymmetricKeyAlgorithm.Plaintext: 0>

I can't find any reference on the documentation or the internet whatsoever and I'm totally lost. The passhphrase is a typical python string, it should be correct, even if it was wrong it should give a different error, I think the problem might reside with the key I loaded (which I was given and am not sure how it was created).

Please can anybody help me with any suggestion?