Help with REST API Bonita studio 7.1

Help please.
I'm working on bonita studio and bonita server, and i''m trying to create a web application with a workflow.
My problem is that i'm using the REST API service and i'm using the method : Update activity variables for bonita 7.1

More specific, the first that i do, its get all the variables that exist in my activity:
http://webDirection/bonita/API/bpm/caseVariable?p=0&c=10&f=case_id%3d
and i have the next response:
{
"description": ""
"name": "nextStep"
"value": "false"
"case_id": "19"
"type": "java.lang.Boolean"
}

then, i want to update the value of the nextStep variable and change his value to true, so i use the update activity variables method as specified in the following direction:
http://documentation.bonitasoft.com/bpm-api-849

so, i use this method:
http://direccionWeb/bonita/API/bpm/activity/
and i use this request payload:
{"variables":"[{\"name\":\"nextStep\",\"value\":\"true\"}]"}
all this using PUT.

The problem i have now is that doing the aforementioned, and i have the next answer: 404 Not Found.
Please somebody help my with this case.
Thanks.

p.s. sorry for my bad english.