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This is the RAW in D&D and this is how it should be in BG3. Please FIX IT Larian. Thank you.
You are plain wrong.
"The gist of Two-Weapon Fighting is this, you can use a bonus action to attack with a light one-handed melee weapon. You can only make this bonus action attack after you take the Attack action with a light one-handed melee weapon in your main hand."
A rapier is NOT a light weapon.
Two short swords work. Two light hammers work. Two hand crossbows work. A combination of any of these works. Like a short sword and a hand crossbow.
Your rapier and dagger does NOT.
There is an exception. The feat 'dual wielder' allows you to use weapons even if they are not light.