When i click on the mega evolve button or z-move buttom, a red massage "this can't be used is this battle" appear. This issue happens in both wild pokemon battle and npc battle. Pls help

You may experiance another issue where you can't use the z move. Just so you don't worry about it and have to wait for a response, below is the fix in case it happens:
By default, Mega/Z moves are used in the Ultra world as this is a gen 7 dimension, however, in all other worlds, dynamaxing is default. This may cause you to not be able to use mega or z-moves correctly.
There is a way to change this to make it so mega/z moves are the default thing, you do this by finding the "pixelmon.hocon" file in your config foldxer and editing the following setting (be sure to close your game first):

# Which dimensions use Gen 7 mechanics by default. Otherwise, they'll use Gen 8.
oldGenDimensions=[
72

Put a 0 under 72 (and put a comma after the 72).
72,
0

Once this is done, save the file and re-launch the game and Z moves should be working as intended.

oh thanks : D