Can't add certificate to 'My certificates' in keychain

Hi everyone,

I bought myself a new notebook. Then I transfered all the data from my previous macbook to the new one by using the

migration assistant. After that, I created a new certificate (Apple Push Services) in the Apple developer portal. Using the keychain, I ceated a certificate signing request.

Now my actual problem: I downloaded the .cer file. Whenever I do a doubleclick on it, the certificate is not imported into the 'My certificates' section in my keychain tool. But I need to export the p12 file, so how can I do that? The 'My certificates' section is completely empty and I am struggling a lot with this issue.

OS version: Catalina, 10.15.2

Can anyone help me, please?

Thanks