The root cause of the issue may be due to your board region not having a stackup defined.
In the Properties panel, make sure that the Selection Filter is set to enable for All - On.
Make sure that the Multilayer layer is enabled and is the active Layer in the layer bar at the bottom. You can confirm that it is enabled by: Select the Panel tab (in the bottom right) ► View Configurations. From here, scroll down to Other Layers and there should be an eye icon next to Multilayer, make sure that it is not crossed out.
After doing so, enter Board Planning Mode by using View ► Board Planning Mode (shortcut: press 1 key).
Then at the bottom in the Active Layers bar, select the Multilayer layer to make it Active.
From here, you should be able to select the Board region. Double-click the board region to open the Board Region properties, and then select the Layer Stack Drop-down, make sure it is set to a Board Layer Stack.