Hello there @digitalsc4rz!
Checking around the community, this issue pops up from time to time. Since you are using WP, the first thing to check is that you are working in the same level tile that your foliage is contained. Or if you are using a persistent level, along with sub-levels.
Once you make sure you are in the same layer as your foliage, try deleting again. If the problem persists, open your foliage assets and double-check that Nanite is disabled for it.
One last thing to check, although it sounds a bit contradictory, in your Erase tool, set “Erase Density” to 0. For further details, please check the related thread below: