There is an implementation restriction on OpenActivities.

I"ve talked about these implementation restrictions before.  Look here: https://community.skuid.com/t/implementation-restriction-on-openactivities .  This is not somthing you are going to be able to get around with Skuid. 

Thanks for responding.

I read this link before posting my query. 

In my page two models and one PopUP(this is used to create new task)
1) Object and Object History.
I granted view all and modify all to user for particular object.

I tried by removing the PoPUP also but still i am facing. How to resolve this.

If you are creating a new task - do not use the OpenActivities sObject.  Use the task sObject.  You should be able to query other sObjects toward prepopulating data in your task, but if its OpenActivities - you can only query a single record.