Cant spawn a block under the player after i send them to custom dimension (its all air, no blocks for now)

I run a procedure to teleport the player to my custom dimension, which as stated is nothing but air. Problem is, i kinda need to spawn a block (preferably a barrier block) in order to kinda stop the player from. oh idk, getting asphyxiated and all of their internal organs becoming external in the void. Problem is, no matter what i do i cant get a block to spawn under the player after the first procedure teleports the player to my dimension.