the fluid brush tool is not working

There's some minimum hardware requirements for this tool.

Your computer must to have a DirectX12 compliant graphics card with at least feature level 12_0 to work. This could be an issue with older cards but any GPU from 2016 onwards should support it.

Here's how to check:

1. Open the Run command. You can do this by holding down the Windows + R keys on your keyboard or searching for Run in your system.
2. Type dxdiag in the dialogue box.
3. Click OK or press Enter. This will open the DirectX Diagnostic Tool.
4. Change to the Display tab. If you have multiple monitors connected to your system, you may have several tabs. Make sure that you select the one that is connected to your dedicated graphics card.
5. Locate Feature Levels: in the Drivers section.

If feature level 12_0 isn't listed it means it's unsupported.