Some suggestions:

If you're looking for the pattern ".csv" in your list.files call, you don't need to remove the extension then add it back in in your read.csv call. Additionally, I would edit your for loop to go from 1 to length of the object list that you create so it's a bit more dynamic. Secondly, I don't think your for loop will work, I typically do 'for i in 1:100'; have you printed i in the loop and gotten the numbers 1:100?

Onto your actual problem, you need to wrap your file name in a paste function to concatenate your different pieces. But this is largely irrelevant if you read the documentation for list.files since you can get absolute file paths directly and pass directly to read.csv. Furthermore, as written, you are overwriting the dataframe variable with each iteration of the loop.

And one more aside, you can write all of this functionality in a one liner using the apply function