One more note for future reference - try checking the output of $FWDIR/log/api.elg.
Address: http://127.0.0.1:50276/web_api/install-policy
Encoding: ISO-8859-1
Http-Method: POST
Content-Type: application/json
Headers: {Accept=[application/json], connection=[keep-alive], Content-Length=[91], content-type=[application/json], Host=[127.0.0.1:50276], User-Agent=[mgmt_cli], X-chkp-sid=[1KtvuY0yiF_JFFSkhc4o2MJZBcS_q7D4AnzMUQfpHS8], X-Forwarded-For=[127.0.0.1], X-Forwarded-Host=[127.0.0.1], X-Forwarded-Host-Port=[443], X-Forwarded-Server=[192.168.19.131]}
Payload: {"Standard":"Standard1","access":"access","policy-package":"policy-package","true":"true"}
--------------------------------------
2017-08-23 11:54:14,088 INFO com.checkpoint.management.web_api_is.utils.helpers.ApiCache.<init>:25 [qtp-27716996-27] - Cache created and initialized
2017-08-23 11:54:14,089 ERROR com.checkpoint.management.web_api.utils.WebApiCommandExceptionUtils.getErrorReply:219 [qtp-27716996-27] -
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "Standard" (class com.checkpoint.management.web_api.core.handler.commands.on_targets.install_policy.PolicyInstallationRequest), not marked as ignorable (8 known properties: "install-on-each-target-of-the-same-version-independently", "prepare-only", "targets", "threat-prevention", "access", "install-on-all-cluster-members-or-fail", "policy-package", "revision"])
at [Source: {"Standard":"Standard1","access":"access","policy-package":"policy-package","true":"true"}
; line: 1, column: 14] (through reference chain: com.checkpoint.management.web_api.core.handler.commands.on_targets.install_policy.PolicyInstallationRequest["Standard"])
...
...
...
---------------------------
ID: 32
Response-Code: 400
Content-Type: application/json
Headers: {Content-Type=[application/json], Date=[Wed, 23 Aug 2017 08:54:14 GMT]}
Payload: {
"code" : "generic_err_invalid_parameter_name",
"message" : "Unrecognized parameter [Standard]"