How to mask beadboard? My whole upper floor including the stair well

is the wood floor to ceiling or done as half wall? If it is a half wall, you could paint the walls in the bedrooms one solid colour floor to ceiling to soften the effect

Is that a light yellow on the walls? I was thinking more white/ blue accents in your furnishings would help break up the flow of beadboard. The dark furnishings stand out in contrast that it stops the eye. Paint your picture frames and hand white flowy drapes between doorways. Add some blue/yellow/white throw cushions. Would be more of a cottage feel. As posted above, you can paint the walls to change it up a bit.

wallpaper liner

Since I see a sewing machine in your photos you must be a creative person with a lot of fabric in your house. Why not cover parts of the wall with fabric. It won't damage the wood and can be easily changed. You could sew a hem along any edges that would be visible and simply tack it along the top and bottom pulling it smooth as you go. You can use hook-and-loop tape, also. Or, you could hang it softly like a drapery and hem the edges putting grommets along the top.

Wallpaper over it. The heavier the wallpaper the better. I did this over wood paneling and it totally transformed the room.

It's a lot of work but use spackle or wood putty to fill in the dips, sand, prime and paint. I just did my wainscoting and you can no longer see them. It will take at least two rounds of putty since it srinks some.

do not wall paper over this beautiful wall, paint is your best friend, like the others stated use throw pillows, nice white curtains, furniture makes it dark

Framing the existing bead board with planks dresses it up. If that is not an option I would paint white and maybe do a chair rail mural to break it up.