Hello
First of all you do not say make or model of the radio?
Most Vauxhalls (I have a Vectra) When new were supplied with a handbook that covers the standard ones. R100, CAR200,300, 400, CCR600, 800 and CDR500. Admittedly mine is a 1997 but none of these have removable front panels. They are all coded radios. As far as I know any radio fitted after that will also be the newer coded type.
So this may be a owner fitted one, Having removed the 4 screws they will reveal 4 holes, The removing tool (available from Halfords) is 2 U shaped pieces of wire like coat hanger wire. Normal ones will fit 1 each side (Top and Bottom) holes and pushing in the holes releases the whole radio.
Yours is saying "Panel" This normally means you have a radio with a removable Front Panel. (Older ones used to have part of the Panel removable, Newer ones you remove the whols Front Panel but these tend to have the 4 screws behind the Front Panel. This was a security feature as the radio is coded to Front Panel supplied only. The request for Panel means it has not made contact properly with the Panel.If you do not have the hand book for the radio try Goggling the Make and Model. Should be a small button to press to release. If so clean electrical contacts and refit. You do not have to enter a code when refitted.
As it reads "Panel" this means the rear electrical contacts are OK as it is displaying a message.
Hope this helps
Andy C