Get rid of this
string splitStrings = originalString.Split(' ');
int test = 0;
bool result = int.TryParse(splitStrings, out test);
You are testing if an array of strings can be parsed to an int, which it can't, that code won't even compile. Splitting on space then checking if it will parse to int word by word won't work anyway as when it gets to the 8.00 the "word" will be "8.00," which won't convert to int (or decimal\double etc).
If you read the question a little more carefully;
reversing only the alphabets [a-zA-Z]
then look at your if condition;
if (originalString[i] == ' ' || i == originalString.Length - 1 || result)
First off get rid of "result" as that's now gone (see opening comment), and checking for ' ' doesn't really match the requirements. What you want to do instead is check if it is a letter (a-z) and if it is add it to charlist, and if it isn't add the reverse of charlist to reverseWordString. Updating the "if" to
if (!char.IsLetter(originalString[i]) || i == originalString.Length - 1)
will get you started, then all you need to do is amend the logic inside that "if" statement to make it simpler really, all it has to do is add the reverse of charlist then add the character that isn't a letter.