Ruler and Grid print functions available in Lightroom?

There is a method (a 'work-around' )  that can use a PNG image file to overlay your photo .

If You can construct an image that has grid lines on it, you can overlay this grid on any image as an Identity plate in the Print module of Lightroom, either to just view it in Lr, or Print to a printer or a  jpg file that you can use any way you wish.

If you have Photoshop, then with several steps you can create a grid image. (You only need to do this once. you have the grid for all time in the fashion of a "template".)

1. Open a new 12x16in document.

2. Define and Show visual guides for 16 column 12 rows (Landscape)  Menu > View> New Guide Layout

3. Add a new blank layer and use Brush to draw lines on this blank layer using the guides to place the lines.

4. Unlock the Background layer and delete it, leaving only the transparent layer with drawn lines.

5. Save the document as a PNG type file to preserve transparency. (eg.  12x16in Grid ID Plate.png )

In Lightroom Print Module-

1. Select your photo that is cropped  12:16

2. Tick to add an identity plate, select the "Graphic" option and choose the PNG file you saved from Photoshop

3. Click on the Identity plate and drag the corners to exactly cover your photo. (it does not automatically re-size to your photo)

4. View, Print, or save as jpg.

If you do not use Photoshop, another editor might be able to create the Grid, but it MUST have transparency between the lines, otherwise you would not see your photo 'under' the 'Grid' Identity plate.