There is a bolt screwed inside a long hole:
I don't know the bolt head type. Is there a trick to open it?
What’s this bolt head type and how can I open it
boltsscrewdriverscrews
Related Solutions
It's called a spanner slotted (or slotted spanner) head. Useful site here. Hard to find in the UK, why the owner used one I cannot imagine. Screwfix doesn't have them. This site may do, although it is international. You'll need to figure out the correct size before you order.
Alternatively get a real cheap flat-head screwdriver the right width and file down the middle until you get the right notch.
If you can grip it with a pliers try a pair of locking pliers and unscrewing it that way I keep a mini vise-grips handy for that sort of thing.
Lock nuts and carriage bolts are a bad combination and even worse when trying to use them in material like particle board or MDF.
You could try to use a Tee nut from the monitor side and then simply use a short hex head bolt into the Tee nut.
Best Answer
It looks like a spanner tamper-proof security bolt. Either buy a set of tamper-proof security bits, or modify a cheap standard slotted screwdriver with a dremel tool. Here is a cheap set: https://www.amazon.com/Performance-Tool-W8659-Security-32-Piece/dp/B002KS19PK/ A good chance that something in this set will work, but no guarantee.