Minecraft – Unable to summon object with this command

The coordinates are pointing to unloaded chunks. You will either need to load those chunks or change the coordinates to point to loaded chunks.

If you were intending to summon the skeleton relative to the command sender's location, you will want to use ~ to indicate relative coordinates as you've done for the Y value.

summon Skeleton ~ ~ ~ {Equipment:[{id:minecraft:bow},{id:minecraft:diamond_boots},{id:minecraft:diamond_leggings},{id:minecraft:diamond_chestplate},{id:minecraft:diamond_helmet},AbsorptionAmount:20,Riding:{id:"Bat",ActiveEffects:[{Id:14,Amplifier:1,Duration:999999,Ambient:1}]}}

Using the execute command is the correct way to do this, but you can not use nbt data in the execute command.

From the command wiki the execute command syntax:

execute <entity> <x> <y> <z> <command …>

There is no argument for data tag information with the execute command. This is why you got the error that stated {display:{Name:example}} is not a number. Look at testfor to see a command with the data tag argument. You will have to use a different method of targeting the entity within the execute command.

This solution uses four command blocks. These command blocks need to be in loaded chunks in order to function properly. If placed in the spawn chunks, they will always remain loaded.

I also recommend you silence command block output to prevent chat spam. Use command:

/gamerule commandBlockOutput false

The first command block adds the tag HoldingMoo to the player holding a snowball named Moo. It is Repeat Unconditional Always Active and its command is:

scoreboard players tag @a add HoldingMoo {SelectedItem:{id:"minecraft:snowball",tag:{display:{Name:"Moo"}}}}

The second command block adds the tag Moo to any snowball within a radius of 3 from a player tagged HoldingMoo. It is Chain Conditional Always Active and its command is:

execute @a[tag=HoldingMoo] ~ ~ ~ scoreboard players tag @e[type=snowball,r=3] add Moo

The third command block removes the tag HoldingMoo from all players. This is for when a player is no longer holding the named snowball. It is Chain Unconditional Always Active and its command is:

scoreboard players tag @a remove HoldingMoo

The final command block is not apart of the chain. It is where you run the summon command. It is Repeat Unconditional Always Active and its command is:

execute @e[tag=Moo] ~ ~ ~ summon cow ~ ~10 ~

You could build a second copy of the same setup with a different set of tags for a different named snowball to do different things. I built a second set up using the tags HoldingBolt and Bolt to summon lightning. Here is the command I ran in the last command block:

execute @e[tag=Bolt] ~ ~ ~ summon lightning_bolt ~ ~ ~

Here is a picture of the cow carnage:

Here is a picture of the lightning snowball:

This solution is simple and will work for most needs but is not perfect. If a player were holding a snowball named Moo and another player threw a snowball at the first player, it might summon a few cows.

If you would like to be more specific, you can create an objective using useitem which will add one to the score every time a player throws a snowball. This will only tag a snowball within the radius of a player who is both holding a custom named snowball and has thrown a snowball.

Create the objective manually by typing this command into chat:

/scoreboard objectives add Thrown stat.useItem.minecraft.snowball

Then be more specific with the target selector by changing the first command block command. This will now only add the tag HoldingMoo to a player who is holding a snowball named Moo and has thrown a snowball. The modified command:

scoreboard players tag @a[score_Thrown_min=1] add HoldingMoo {SelectedItem:{id:"minecraft:snowball",tag:{display:{Name:"Moo"}}}}

This method will require one more command block on the chain, to make it four long. This extra command resets the score tracking how many snowballs each player has thrown. This command block is Chain Unconditional Always Active and its command is:

scoreboard players reset @a Thrown