I'm assuming you're not on the last phase. It's not bugged.
The first time he throws it, you have to use a special attack into it. Then you get stacks. Once you have stacks, you can either parry or heavy attack into it.
You need to have the green removal charge first. Special into his blast to get it then you can parry the next blast.
What Rafikii said, you have to throw a special to get a removal charge, looks like a green icon in your HUD
thanks that solves the issue
Just a general tip on any boss fight like this, pause the game and it will usually tell you what's going on or what you need to do. In this case it will tell you what to do based on the phase (throw heavy or special into plasma wave then knock down).