Cannot remove double vertices I tried to follow a tutorial , which showed to remove double vertices in blender by pressing F3 and typing "Remove double vertices" but I just can't find that option to remove those extra vertices.

Also i tried with the Merge By Distance option but i still see 13 vertices where it should be 9 vertices.

I want to remove double vertices between the triangle plane and the top of the cube

Please help  Sir,
Merge by distance should fix your problem, but you need to select the entire model. If it still doesn't work try increasing the merge distance.

Try identifying of which vertices you have duplicates by using the box select tool on one of them and paying attention to the bottom left of the window. If it says "Verts: 2/13" you press "alt + m" and click on "at center". Repeat this until all duplicates are gone.