I made a union out of different parts
I can walk through everything except on one place…
What did I do wrong?
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That often a problem for developers new to unions. I suggest “Grouping” the parts instead.
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I tried it but that just makes it worse…
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Please clarify what you mean by “worse”.
now there are more pieces that i can’t go through
It’s not a “problem” but a normal thing. See, Unioning parts technically turns them into one big part and the chipped off parts will certainly always have this effect. I suggest leaving the stairs just as they are pre-unioning.
the stairs arent a union I just added blocks on the stairs so it would make it transparant where I can walk through
Hold Alt and click on the area that your character hovers above to select it, and then move/disable CanCollide to fix it. If neither of those will work, and it’s a union, you’ll have to un-union it.
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Generally you want to avoid using unions on large objects. I would suggest instead of unions to build your stairs just use parts parts to make those stairs which you can easily do.
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I’ve problems with collisions similar to this before. IIRC, collisions are simplified when unioning parts. This is to make things easier and improve performance. The simplest solution I can think of is to export the union as a .obj file and import it back into the game as a mesh.
This is a common problem with building, In matters like this see if there is any invisible bricks if not ununion everything and group it, You could also try to turn some peaces into mesh’s if you wanted to try that.
When you union parts, the physics may be altered. This has happened to me many times too. (As the builds that I worked upon were a combination of mesh, Roblox parts and unions I was unable to just separate the unions to solve the issue.) This is how I have overcome this problem:
(1) I have set the collisions of the main build to ‘false.’
(2) I have constructed a shell of simple Roblox parts that mimic the physics that I wish to employ. I have coloured these red so that you can see them easier in the image below.
(3) I have set the collisions of these additional parts to ‘true.’
(4) Finally, I have set the transparency of these additional parts to 100% so that you are unable to see them. This is what the final build then looks like. (To show that the ‘red parts’ are still present in the final image, I have selected them. You can see them highlighted with a faint blue outline.)
Good luck in solving this physics issue. I will look forward to seeing your final build. 
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Thanks for all the help but I think that the comment of riverblocks is the best solution!
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