Minecraft – How to summon an invisible falling block in minecraft 1.16.5

Try summoning a Minecart Chest riding placeholder entity.

/summon MinecartChest ~ ~-1 ~ {CustomName:"Hidden Chest",Riding:{id:Chicken,Invulnerable:1,NoAI:1,Silent:1,Age:-2147483648,ActiveEffects:[{Id:14,Amplifier:0,Duration:2147483647,ShowParticles:0b}]}}

Note: This command is too long to paste into chat, use command block instead.

Added information about Invincibility - See Making mobs invincible (David Toh's solution for more details) and http://minecraft.gamepedia.com/Chunk_format#Entity_format.

There is no n parameter. The correct parameter you're looking for is name. The wiki has a list of all valid parameters here.

testfor @p[1303,56,518,2,name=JAMES_the_camel]

For 1.11-1.12, which removes shorthand coordinates:

testfor @p[x=1303,y=56,z=518,r=2,name=JAMES_the_camel]

And for 1.13+, which overhauled the command system (where /testfor has essentially been merged into /execute):

execute if entity @p[x=1303,y=56,z=518,distance=..2,name=JAMES_the_camel]

As well, because your chain is unconditional, it's going to activate no matter what and open the door. If you set it to conditional, it will only run its command if the command block physically behind it was successful. You will want to set your chain block to conditional or otherwise activate another set of command blocks based on the success of /testfor.