From what I could debug, your code breaks when a key of the dictionary is repeated. So, the max value of the dictionary is not of the whole string, it is from a part of it.
Commenting the break line out seems to fix it.

If you don't use backticks (to format code here), some things will dissapear from your code.

Dictionary score = new Dictionary(); I'm pretty sure the data types are missing there.

Yes really. Could you check and try using code block formatting?

https://ibb.co/h9ag28

Also, which libraries are you using?, maybe the error comes from that missing part.

It's giving me a lot of errors even in sample tests, take a look at the link I posted there on how to use markdown formatting just in case something went missing there.

See this, paste your code here and mark your post as having spoiler content. Also when you do a kata that has several languages like this one, it's useful to say which language you're having problems with.

You need to return the highest scoring word as a string.

So, yes, you have to return "fdhghsdfdsfdsf" in that case.

Proof?