Removing the dark glitches on a mesh in Blender

[CriticalDucky]

I imported a waterslide with tons of wedges from Roblox to Blender, and I was shown this:

This is how it looks like in Studio and how it should look like in blender:

I tried merging the vertices and changing it to smooth and flat shading. How do I fix this?

[GonderDex]

Click on ‘Clear Custom Split Normals’
And also remove the faces between where theyre merged