I was trying to configure push notifications from Safari on Control Panel but I
have an error: "Failed to convert pem into der"
This error message is displayed if you try to upload the certificate itself, not the private key exported from it. Please note that for Safari you have to upload your Private Key which you export from the certificate in the *.p12 format.
Please refer to Exporting Private Key part of the iOS Configuration Guide:
If you've done all these steps and the .p12 file is not uploading, please check whether your .p12 file contains both Private Key and the certificate. If the certificate is missing for some reason that might be the issue.
In that case, try to create a new certificate (as described in the Safari Guide) with your current Website Push ID, and then export the new .p12 file.