I would like to check the block left to player and spawns an entity when it is an iron block. How can I do so?

first the Player's left Block depends on his facing:

• when the Player faces north the left Blocks has a offset with x: -1, y: 0, z: 0
• when the Player faces east the left Blocks has a offset with x: 1, y: 0, z: -1
• when the Player faces south the left Blocks has a offset with x: 1, y: 0, z: 0
• when the Player faces west the left Blocks has a offset with x: 0, y: 0, z: 1

 you need to add the offset to the current position of the Player and then
check via Level#getBlockState and BlockState#is if the Block is Blocks.IRON_BLOCK