I will assume originally two switches, I suspect you will need a dimmer switch that does not require a neutral. I don't thing you can use the switch you have.
Warning LED bulbs and neutral less electronic switches cause problems, with a standard BA22d, E27 or E14 lamp a dimmer not needing a neutral may work.
Less likely with GU10.
And highly unlikely to work with G9 LED bulbs.
It is unlikely the black wires shown are neutrals, they are more likely the switched line, I can't see an earth, so likely wiring pre-1966, and the house likely needs a rewire, and until rewired no metal lights or switches should be used.
Or to put it another way. If you look at your new switch, the first terminal on the left is marked as N, That means neutral. The cabling you have does not have a neutral, so that means the switch you have will not work with the wiring you have. It is quite a common mistake, and short of rewiring there is nothing you can do about it. Also as pointed out, your cables do not have an earth wire, this in turn means you should not use any light fittings (or switches) that require an earth. It would be a good idea to have your house electrically inspected (EICR) or better still completely rewired.
The cables you have although they are coloured red and black which most folk would assume to be live and neutral, this is not the case with light switches, red is live, but black is switched live, not neutral.
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