FATAL_ERROR System.DmlException: Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, You cannot create a link for this type of entity through the api: Linked Entity ID: [LinkedEntityId]

trigger TrgAttachment on Attachment (after insert) {
    System.debug('TRG ' + Trigger.New[0]);
    Attachment att = Trigger.New[0];
    //Task t = Trigger.New[0];    
    System.debug('TYPE ' + att.ParentId.getSobjectType());
    Task t = [SELECT Subject, WhatId FROM Task WHERE id=:att.ParentId][0];
    
    System.debug('ATT ' + att);
    if(t.WhatId <> NULL && t.WhatId.getSobjectType() == opportunity.getSObjectType()){

        Opportunity o = [SELECT Id, Name FROM Opportunity WHERE id=:t.WhatId][0];
        System.debug('Related to opps' + o);
        
        ContentVersion cv = new ContentVersion();
        cv.ContentLocation = 'S';
        cv.PathOnClient = att.Name;
        cv.Origin = 'H';
        cv.OwnerId = att.OwnerId;
        cv.Title = att.Name;
        cv.VersionData = att.Body;
        insert cv;
        
        ContentVersion cv1 = [select Id, ContentDocumentId from ContentVersion where Id =: cv.Id][0];
        Id linkDoc = o.Id;
        ContentDocumentLink cl = new ContentDocumentLink();
        cl.LinkedEntityId = linkDoc;
        cl.ContentDocumentId = cv1.ContentDocumentId;
        cl.ShareType = 'V';
        try{
            insert cl;
        }
        catch(Exception ex){
            throw ex;
        }

        
    }
}