public class SlackOpportunityPublisher {
    private static final String slackURL = 'https://hooks.slack.com/services/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';
    
    public class Id {
        @InvocableVariable(label='Opportunity Name')
        public String opptyName;
        @InvocableVariable(label='Type')
        public String Type;
        @InvocableVariable(label='Stage')
        public String stage;
        @InvocableVariable(label='Probability')
        public String probability;
        @InvocableVariable(label='First')
        public string first;
        @InvocableVariable(label='Last')
        public string last;
        @InvocableVariable(label='Close')
        public string close;                 
    }
    @InvocableMethod(label='Post to Slack')
    public static void postToSlack(List<Id> opportunityId) {
        Id oppId = opportunityId[0]; // If bulk, only post first to avoid overloading Slack channel
        Opportunity opportunity = [SELECT Name, Type, StageName, Probability, Owner.FirstName, Owner.LastName, CloseDate from Opportunity WHERE id=:OPPid];
        Map<String,Object> msg = new Map<String,Object>();
        msg.put('text', 'The following opportunity has changed:\n' + 'Account Name And Product: ' + Opportunity.Name + '\nType: ' + opportunity.Type + '\nNew Stage: ' + opportunity.StageName + '\nSales Person: ' + opportunity.Owner.FirstName + ' ' + opportunity.Owner.LastName + '\nProbability: ' + opportunity.Probability + '%' + '\nForcasted Close Date: ' + opportunity.CloseDate);
        msg.put('mrkdwn', true);
        String body = JSON.serialize(msg);    
        System.enqueueJob(new QueueableSlackCall(slackURL, 'POST', body));
    }
         
    public class QueueableSlackCall implements System.Queueable, Database.AllowsCallouts {
             
        private final String url;
        private final String method;
        private final String body;
             
        public QueueableSlackCall(String url, String method, String body) {
            this.url = url;
            this.method = method;
            this.body = body;
        }
             
        public void execute(System.QueueableContext ctx) {
            HttpRequest req = new HttpRequest();
            req.setEndpoint(url);
            req.setMethod(method);
            req.setBody(body);
            Http http = new Http();
            if (!Test.isRunningTest()) { // HTTP callout is not allowed in tests apparently...
            HttpResponse res = http.send(req);
        }
    }
    }
}
@isTest
private class SlackOpportunityPublisherTest{
    static testMethod void testPostToSlack() {
        SlackOpportunityPublisher.Id pubTest = new SlackOpportunityPublisher.Id();
        pubTest.opptyName = 'Test opportunity';
        pubTest.type = 'Test Type';
        pubTest.stage = 'Test stage';
        pubtest.probability = 'Test probability';
        pubTest.first = 'Test First';
        pubTest.last = 'Test Last';
        pubTest.close = 'Test Close';
        List<SlackOpportunityPublisher.Id> theList = new List<SlackOpportunityPublisher.Id>();
        theList.add(pubTest);
        SlackOpportunityPublisher.postToSlack(theList);
        System.assertEquals(theList, theList); // Can't really test this, just put something that is true
    }
}
Any help is appriciated this is the problem Method does not exist or incorrect signature: void postToSlack(List<SlackOpportunityPublisher.Id>) from the type SlackOpportunityPublisher