I am working to make a border for a GIF. May I ask please how to enable the stroke selection for a new layer in GIMP?
As far as I can tell from your screen shot, you haven't made a selection. Stroke selection will be greyed out if you don't have a selection.
The yellowish dotted line around your image is not a selection, it's only the layer boundary.
A selection looks different from the layer boundary, it has a "marching ants" animated effect, and is not yellowish.
You can switch off the layer boundary if it's confusing you. Click View in the main menu, and deselect the Show Layer Boundary option.
If you want to select the entire image hit Ctrl+A(Select all). Now you can stroke it using Stroke Selection.