Overview:
Error: You are not allowed to schedule this type of event during this time frame. Please contact your administrator. displays when creating child Events in CRM Online.

Root Cause:
One possbile reason is that the end-user does not have access to the Child Event Record Type(Child_Event_Record_Type_vod__c) field in the Event Rule(EM_Event_Rule_vod__c) object.
Solution:
Be sure the field level security(FLS) of the Child Event Record Type(Child_Event_Record_Type_vod__c) field in the Event Rule(EM_Event_Rule_vod__c) object is granted to the end-user.
Related Documentation:
CRM Help Documentation: Create Child Events from a Parent Event
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