How can I make python script show an error when user inputs same file name with this piece of code?

You'd want to check if the file exists after they input it, which can be done using "os.path.exists" as also suggested by Tim Roberts. Here's code that should work for you:

elif 'note' in query:
        try:
            Looper = True # Added a loop so it can repeatedly ask the user until they give a valid answer.
            speak(" Okay master. what's the file name") # Put here instead of in the loop so it will repeat once since the else condition already asks for a new file name aswell.
            while Looper == True:
                b = takecommand()
                if os.path.exists(f"{b}.txt"):
                    f = open(f"{b}.txt","w")
                    speak("Okay master. tell me what to note down")
                    a= takecommand()
                    f.write(f"{a}\n")
                    Looper = False # So the loop won't repeat again (regardless of the below line).
                    f.close()
                else:
                    speak("That file already exists. Give me a new file name.")
        except Exception as e:
            speak("Hm, an error occured. Try again later.")

I also added in a while loop for you, so that way if the user gives a file name that already exists, it will keep asking until they give a valid file name.

Ensure you have OS imported for the code to work by adding the following to the top of your code:

import os

Use mode 'x'

x' create a new file and open it for writing. The 'x' mode implies 'w' and raises an FileExistsError if the file already exists.

try:
    # ...
    f = open(f"{b}.txt","x")
    # ...
except FileExistsError:
    speak("file name already exist")

Or mode 'a' to append the strings to existing file.

'a' open for writing, appending to the end of the file if it exists