Error creating an appointment

Hi

I tried to create an Appointment and i get this error

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Principal user (Id=b566be81-adbd-e211-ba94-b4b52f67d656, type=8) is missing prvShareActivity privilege (Id=b5f2ee06-d359-4495-bbda-312aae1c6b1e)Detail:

<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">

 <ErrorCode>-2147220960</ErrorCode>

 <ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />

 <Message>Principal user (Id=b566be81-adbd-e211-ba94-b4b52f67d656, type=8) is missing prvShareActivity privilege (Id=b5f2ee06-d359-4495-bbda-312aae1c6b1e)</Message>

 <Timestamp>2013-05-16T22:24:21.1104048Z</Timestamp>

 <InnerFault i:nil="true" />

 <TraceText i:nil="true" />

</OrganizationServiceFault>

I can create a "phone call", "task", etc. But not an appointment. What can i do?