I am trying to add  existing contacts to a related list on Opportunities  as "Job Candidates" through a VF page and kicked off by a custom button. I keep getting the compile Error  Error: Compile Error: Invalid field OpportunityId for SObject Contact at line 35 column 75 .  The functionailty is suppose to search contacts (which i should make for specific record type of "participant"  and "Participant Donor" just haven't gotten there yet. 

What am I missing? 

public with sharing class AddContacttoJobOpptController { 
    public Opportunity opp {get;set;}
      public String selectedConId = '';
    public String selectedOppId = '';
    public AddContacttoJobOpptController(){
        opp = new Opportunity();
        selectedOppId = Apexpages.currentPage().getParameters().get('aid');
            
        
    }
    public Pagereference addContact(){
        Set<String> conIdSet = new Set<String>();
        selectedConId = opp.Candidate__c;
        if(selectedConId != null && selectedConId != ''){
            for(Contact con : [select id from Contact where OpportunityId = :selectedOppId]){
                conIdSet.add(con.id);
            }
            if(conIdSet.contains(selectedConId)){
                apexpages.Message msg = new Apexpages.Message(ApexPages.Severity.Info,'The selected contact is already added to strategic account plan. Please select another contact to add.');
                apexpages.addmessage(msg);
                return null;
            }
            else{
                Contact con = new Contact(id=selectedConId, OpportunityId=selectedOppId);
                update con;
                apexpages.Message msg = new Apexpages.Message(ApexPages.Severity.Info,'The selected contact has been added to strategic account plan. Now you can select another contact to add.');
                apexpages.addmessage(msg);
                return null;
            }
            
        }
        else{
            apexpages.Message msg = new Apexpages.Message(ApexPages.Severity.Info,'Please select contact then click on Add Contact button.');
            apexpages.addmessage(msg);
            return null;
        }
        
    }
    public Pagereference goBack(){
        return new Pagereference('/'+selectedOppId);
    }

}