There is no user with this username! You can't send messages to someone who isn't there!
Above is a description and possible solution to the Telegram error "Username is invalid".
You, as a user of the program, should understand that this error can occur both suddenly (for example, if your account reaches the Telegram limit) and predictably (for example, if you do not add Telegram account, or if you forgot to add the text of the message).
However, you should remember that the proposed solution to the problem may not always be suitable for you. But you can always find this error on the Internet. Perhaps there you will find a more accurate way to solve your problem.
Also, you need to understand that the author of the Telegram-Spam-Master program may not know how to solve your error. So if in the process of searching you find a solution on your own, it would be very good if you shared this solution with him!