If the keystore is PKCS12 type (.pfx) you have to specify it with -storetype PKCS12 (line breaks added for readability):
keytool -genkey -alias <desired certificate alias>
-keystore <path to keystore.pfx>
-storetype PKCS12
-keyalg RSA
-storepass <password>
-validity 730
-keysize 2048