What is the modification requirement of bullet short message signal? Why do I always fail?

Yes, I have tried many times, and the modification was unsuccessful!

I can't change it on my mobile phone. I use the web version, which shows that there is no chance to change it.

Later, I found that I still need to change it with my mobile phone. Note that "the first letter must be lowercase", followed by letters, numbers and underscores. Change is successful.

Would you like to try?