It is unclear what you are trying to do here, since you only describe the problems you run into and not what task you are trying to implement at the end. Anyway ...
openssl req -text -in certificate.pem
This line expects a certificate request. Your code instead creates a certificate (CertificateBuilder), not a certificate request. The latter would be created with x509.CertificateSigningRequestBuilder, which as expected works with the openssl req command above.
... I get an error when I try to build a self-signed certificate with x509.CertificateBuilder.
It does not look like you get an error when building the self-signed certificate, i.e. the code to build the certificate works. Instead you get an error when using it with openssl req. This error is expected since you did not provide a certificate request but instead a certificate. For certificates use the x509 openssl command not req:
openssl x509 -text -in certificate.pem Check if the first line of your certificate request starts with:
-----BEGIN CERTIFICATE REQUEST----- 
