public static String accessToken(){
    Token__c tok = [SELECT ConsumerKey__c, ClientSecret__c, Username__c, Password__c, SecurityToken__c
                            FROM Token__c
                            WHERE Name = 'OurToken'];
    String consumerKey = tok.ConsumerKey__c;
    String consumerSecret = tok.ClientSecret__c;
    String username = tok.Username__c;
    String password = tok.Password__c + tok.SecurityToken__c;
    String request = 'grant_type=password&client_id=' + consumerKey +'&client_secret=' + consumerSecret +
                     '&username=' + username + '&password='+password;
    return request;
}

@isTest
static void testCallout() {
    Test.setMock(HttpCalloutMock.class, new MockHttpResponseGenerator());

    Http ourHttp = new Http();
    String requestBody = Callout.accessTokenBody();  //error is here
    HttpRequest ourRequest = new HttpRequest();
    ourRequest.setBody(requestBody);
    ourRequest.setMethod('POST');
    ourRequest.setEndpoint(System.Label.Job_Advertisement_URL + '/services/oauth2/token');
    HttpResponse response2 = ourHttp.send(ourRequest);
}