If your TeamViewer won’t connect to your Pi when you don’t have a screen attached.

1. Open raspberry pi config file.

sudo nano /boot/config.txt

2. Use the arrows and go to the end of the file and add this text.

#Always force HDMI output and enable HDMI sound
hdmi_force_hotplug=1
hdmi_drive=2

3. save with CTRL + O 4. Quit with CTRL + X 5. Reboot. Problem solved