put your server declaraition outside the constructor and call a class callback : 



    
    #include <Arduino.h>
    #include <ESP8266WiFi.h>
    #include <ESP8266WebServer.h>
    
    ESP8266WebServer server(80);
    
    // Function to create AccessPoint to get ssid and password
    int Device::getWifiCredentials() {
    server.begin();
        
        // define callbacks
        server.on("/", std::bind(&Device::handleRoot, this); // initial page
    
        server.on("/ssid", std::bind(&Device::handleCredentials, this); // catch response from initial page
    
        // wait for credentials to be delivered
        server.handleClient();
    
        while( WiFi.status() != WL_CONNECTED){
            Serial.print(".");
            delay(500);
        }
    
        return ACTION_SUCCESSFUL;
    }