In Apigee there are multiple levels of reference for TLS keys and certs.
You can cite keystores and TrustStores directly, by name:
...
<KeyStore>my-keystore</KeyStore>
<TrustStore>my-truststore</TrustStore>
...
Or you can use a separate construct, which is a REFERENCE to a keystore or trruststore, this way:
...
<KeyStore>ref://my-keystore-reference</KeyStore>
<TrustStore>ref://my-truststore-reference</TrustStore>
...
See documentation on references here: https://docs.apigee.com/api-platform/system-administration/working-references
You said
imported SAP cert in the keystore.
But your configuration shows that you are using a reference, and you didn't mention configuring the keystore reference. So I think you missed a step. Or maybe you don't need or want to use references. In which case, eliminate the ref:// prefixes in your configuration.
One other thing :
probably your keystore and truststore should not be the same. That's not good TLS hygiene. Please check that. In general it's not a good idea to store private keys in the same place as you store trusted certificates.
Good luck