I cannot add a wireless device (push button, motion sensor, door/window sensor) and the doorbell beeps twice when I try to do so.

You have exceeded the maximum number of wireless devices that your Honeywell Home doorbell can connect to. Your doorbell will accept 2, 4, 5 or 6 devices according to the model. Please refer to this table:

Series 3 (DC311xx and DCR311xx) 2 wireless devices
Series 3 (DC313/5xx and DCR313/5xx) 4 wireless devices
Series 5 (DC515xx) 4 wireless devices
Series 9 (DC915xx and DC917xx) 6 wireless devices
Series 9 wired (DW915xx) 2 wired push buttons and 5 wireless devices

For doorbells with a Halo Light: After attempting to add the new device, wait for the doorbell to beep twice. You now have the option to remove one of the devices already linked to the doorbell, and replace it with the new device. A short press of the Settings button settings will cause the doorbell to indicate the Halo colour of the 1st stored device in the memory. A second short press will cause the Halo colour of the 2nd device to be displayed. Repeated short presses will cycle through the colours for all linked devices. A long press of the Settings button will cause the device indicated by the Halo colour to be overwritten with the new device. There is a 12 second window to complete this operation.

For doorbells with no Halo Light: The Red LED will flash rapidly. A short press of the Settings button settings (less than 2 seconds) will cause the oldest device in the doorbell memory to be overwritten. A long press of the Settings button settings (more than 2 seconds) will cause the newest device to be overwritten. If no button is pressed within 12 seconds, nothing will be changed.

If you are unable to identify the devices because no unique colours for the Halo Light was set, you can perform a factory reset to delete all linked devices from the memory, then link the new device. To do this, remove one battery (or turn off the power to a plug-in doorbell). Press and hold the settings button settings whilst replacing the battery (or turning on the power). An Amber light will flash, either from the small LED indicator on the side of the doorbell, or from the Halo Light (Depending on product type). At this point, you can link a new device.