Minecraft – How to detect a player close to a specific block Minecraft 1.9

The following command should work:

execute @e[type=Slime,name=explore1] ~ ~ ~ spreadplayers ~ ~ 2 3 false @e[type=Slime,rm=0,r=1,name=explore1]

All slimes will try to spread nearby (r=1) slimes that aren't themselves (rm=0) a maximum of 3 blocks around them.

For situations like this, I'd recommend running the command being executed itself (the /fill command) to see what the error is.

The main error appears to be that you're trying to fill a quarter of a million blocks, which is more the maximum (32678) that Minecraft allows for. You could reduce your fill region:

/fill ~-15 ~-15 ~-15 ~15 ~15 ~15 minecraft:diamond_block 0 replace minecraft:gold_block

Or, if the huge area is important, split it up into multiple commands:

/fill ~0 ~0 ~0 ~30 ~30 ~30 minecraft:diamond_block 0 replace minecraft:gold_block
/fill ~0 ~0 ~0 ~-30 ~30 ~30 minecraft:diamond_block 0 replace minecraft:gold_block
etc.

(I'd suggest not running either of these on a clock)

That should be it, but keep in mind that the command will also fail if there are no gold blocks to replace, or if you're too low-down/high-up so that the relative coordinate are selecting an area outside of the world.