What version of the game you are experiencing the issue with:
Classic WOTLK

Describe the bug
I have both blizzard and neatplates castbar overlaped under enemy target life bar.

To Reproduce
Steps to reproduce the behavior:
everytime

Expected behavior
Would like to have only neatplates castbar (much more beautiful !) and not blizzard one for a better visibility

Good job overall !

With a screenshot !
Thx

You can try changing the CVar related to the targets castbar to 0 with this command and see if it does anything
/console showTargetCastbar 0
But I suspect this issue is related to some other addon, that is adding a castbar.

Hi mate !
Indeed it was an other addon (classiccastbar) causing this issue. Problem solved.
Thank you !