Import certificate as PrivateKeyEntry

You try to add certificate and expect that it will be private key - its confusion between two different things.

Generally, when you create keystore (.jks) it include the private key inside. If its empty (deleted) you should generate bundle (.p12 file) from your key and certificates.

In order to create new free key and certificate you can use this this implementation of openSSl https://zerossl.com.

Then, you've got a key and certificate that you should generate (.p12) bundle file from them: (on linux machine)

openssl pkcs12 -export -in [filename-certificate] -inkey [filename-key] -name [host] -out [filename-new-PKCS-12.p12]

Now, just add the bundle file (.p12 file) to a keystore (.jks) by executing the following command:

keytool -importkeystore -deststorepass [password] -destkeystore [filename-new-keystore.jks] -srckeystore [filename-new-PKCS-12.p12] -srcstoretype PKCS12

Get rid of the -trustcacerts option. It isn't a CA certificate. It's your certificate. And use the same alias the private key already had.