Serial Port Will Not Enable

[RWhite]

I need a guest serial port, but I cannot enable one because the Enable Serial Port box in Settings is grayed out. The port is built in to the motherboard, it is enabled in the EFI BIOS, and the port works on the host. Any ideas how fix it so it will enable?
Thanks


Configuration: Host Win10-64 Home, Guest WinXP-32 Home, VM 5.2.6 with Guest Additions.

EDIT: problem fixed itself by turning the computer off for 15 minutes and restarting.
EDIT2: apparently the real issue was that the enabling had to be done with the VM powered off.