Ok, A huge thank you to a twitch user CW390 for figuring this out. He had suggested to use the advance build to get a slab, and well it works! However when you use

I just ran the game with NO MODS, EVERYTHING DEFAULT, meaning: I deleted everything re-installed the game then ran the game with setting Max memory limit to 8192 for 8GB of ram and in game settings like: Full screen, 512 stars, 10000 segments, etc normal game settings, and i still CAN NOT place any slabs.

I checked the over capacity thought, and it still does not let me place any slabs.
I lowered my inventory down to 1.2% cap. and tried to use slabs, but I still get the same error. Also I noticed while having the Allow personal inventory over capacity = true ,the game still told me in red that I am over capacity. Not sure if this is normal or not.