My band is sliding.. I tried to do what written here but still sliding.. my watch new

You didn’t attach what you have already tried so I will have to guess. If the band is not locking correctly into the mounting channel on the watch body please double check it is fitted the correct way. Note particularly the orientation with text toward the back of the watch, locking tabs on the band toward the back of the watch. Picture 2 in Change your Apple Watch band - Apple Support

If you get the band upside down in the mounting it will slide without locking.

you can view how the mechanics work on this image https://www.patentlyapple.com/.a/6a0120a5580826970c01bb086f387e970d-pi

the notches on the band or the locking mechanism connected to the buttons on the back of the watch, are really the only two things which could be culprits, if the plastic notches has been deformed on fractured then the only real solution would be to get a new band