What is the value of i when it stops? And the value of csvs[i]? My guess is you are trying to read in something that is not a csv (or not even a proper file). You can restrict to csv by using the pattern argument of list.files:
csvs <- list.files("C:\\Users\\ M N\\Bachelor\\RCNN_mushroom_test\\RCNN_mushroom_test\\customImages\\val2\\predict_results",
pattern = "*.csv", full.names = T)