When I try to use Flexible block placement in my singleplayer world, it does show the grid. But when i want to place a block on the offset, the block is placed on the original block instead of the corner. And same goes for the block rotation.
I have carpet installed, and i have enabled the AccurateBlockPlacement.

Preferably in single player you shouldn't use Carpet's accurateBlockPlacement rule, but instead let Tweakeroo handle the placement itself. That would be the Generic -> clientPlacementRotation option. But that only affects rotations and other block properties, not the offset feature. If offset isn't working, then I would suspect some mod conflict on the client that prevents any of Tweakeroo's placement related features from working.

I found the mod that made it not work!
it was the clean cut mod. I don't know how and why it makes that so nothing works cause there is like 0 keybinds or stuff linked to the mod (exeppt the use of the left click button).