Do I get to add my dexterity bonus to my damage rolls with Shadow Blade?

Tonight I brought out my shadow blade and my GM ruled that I didn’t get to add any extra damage to my shadow blade from my dexterity, because spells only do what they say they do, which is fine and I rolled with it.

My contention is that the spell says I should be able to add my dexterity to damage rolls since it is a finesse weapon. Was it the intent of this spell to deal 2d8 or 2d8+dex on a hit?

For those who want to know: my GM let me use my spell casting ability + PB for attack rolls.

Shadow Blade says:

You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60).

Finesse says :

When making an attack with a finesse weapon, you use your choice of your Strength or Dexterity modifier for the attack and damage rolls. You must use the same modifier for both rolls.