No great fan of Davies Media, typically more ego than content, however:
Image -> Guides -> New Guide (by Percent) should work. This in a Win7 64 bit (VM) https://i.imgur.com/fSovN5F.jpg
Did it ever work ? Did you have an older (maybe Gimp 2.8) installation ?
Try disabling your Gimp 2.10 profile (by a rename to) C:\Users\"yourname"\AppData\Roaming\GIMP-old
Start Gimp, let it make a new default (empty) Gimp User Profile. See if there is any difference.
Image -> Guides -> New Guide (by Percent) should work. This in a Win7 64 bit (VM) https://i.imgur.com/fSovN5F.jpg
Did it ever work ? Did you have an older (maybe Gimp 2.8) installation ?
Try disabling your Gimp 2.10 profile (by a rename to) C:\Users\"yourname"\AppData\Roaming\GIMP-old
Start Gimp, let it make a new default (empty) Gimp User Profile. See if there is any difference.