Comment 5 for bug 1878553

Manual mode is selected (and consequently other options are greyed out) because the UbuntuDrivers module reports manual_install=True for the device. Diving into the ubuntu-drivers-common code, it invokes "modinfo vboxguest" and if the return value is 0, it considers the module to be manually installed.

ubuntu@groovyvm:~$ modinfo vboxguest
filename: /lib/modules/5.4.0-28-generic/kernel/virtualbox-guest/vboxguest.ko
version: 6.1.6_Ubuntu r137129
license: GPL
description: Oracle VM VirtualBox Guest Additions for Linux Module
author: Oracle Corporation
srcversion: 2A221C27F4AB41327E708B4
alias: pci:v000080EEd0000CAFEsv00000000sd00000000bc*sc*i*
depends:
retpoline: Y
name: vboxguest
vermagic: 5.4.0-28-generic SMP mod_unload
sig_id: PKCS#7
signer: Build time autogenerated kernel key
sig_key: 42:A8:A9:98:8B:E6:F1:71:2C:AF:A3:32:4E:72:D5:45:3C:41:3F:7A
sig_hashalgo: sha512
signature: 41:5D:D4:94:4A:0E:64:89:C4:6C:E8:20:76:5E:37:0B:5C:FB:DF:8A:
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