Dot net DLL signing issue

Signing process
3

It’s working with me when I chnage the private key to PEM format.

-----BEGIN EC PRIVATE KEY-----
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-----END EC PRIVATE KEY-----

X509Certificate (CCSID / PCSID) used for signing is not valid certificate (CCSID / PCSID) for this VAT Registration Number