1. Code Compiled successfully, but not able to upload to NodeMCU :
This problem generally comes up with generic NodeMCU boards. To solve the issue, click on the Open Full Editor option in the Sketch sub-section. You will be redirected to Arduino Create online editor. Click on the dropdown menu available and then on Select Board and Port. In the new window that opens, search for ESP8266 and select Generic ESP8266 Module. Select the port to which the NodeMCU is connected. Now in the Flavours menu, make sure all the specifications indicated match with your NodeMCU board. If everything is correct, then go to the Reset Methods and select nodemcu from the dropdown. Try uploading the code now.
Another possible solution is to use Arduino IDE offline application. As mentioned above, head over to the Full Editor and from the three-dot menu, select Download Sketch option. Once download finishes, extract the .zip file and open the .ino file. Make sure you type in your WiFi details in the arduino_secrets.h file along with the secret key. Verify the device ID in thingProperties.h file. Try to compile and upload now.
2. Code compiled and uploaded successfully, but NodeMCU doesn't connect to WiFi :
A possible issue can be one of these - incorrect WiFi SSID, Password, Device Secret Key, or device ID. Verify all the details in the code. If all the previously mentioned parameters are correct, then check if the WiFi you are trying to connect is 2.4GHz or 5GHz frequency. Most of the NodeMCUs only support 2.4GHz frequency, so verify if the WiFi you provided is of 2.4GHz frequency. Allow atleast 10 seconds for the device to connect to WiFi.
3. Arduino Cloud doesn't detect my NodeMCU :
Firstly, check if Arduino Create Agent is running in the background. If not, then download and open the Arduino Create Agent. Your device must be detected now. If the problem still persists, try uploading a sample code to the NodeMCU using Arduino IDE offline. If the latter is not successful too, then install/update the drivers of the NodeMCU and try again. If nothing from the above works, maybe the NodeMCU is a faulty one. Exchange it for a new device.