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.
Player List
The way that /execute works is not that the player runs the command, but that it sets the player as the effective "command sender", while keeping the command block or the opped user who ran the /execute command as the true command sender. This allows /execute to run op commands that the player could not run, but also means that the player does not see the output of these /execute commands, even if it was targeted to them.
However, you are not out of luck. Using the /tellraw command, you can display any message you want, including a selector.
/tellraw @a ["",{"text":"Welcome to the server! Here is a list of all online players: "},{"selector":"@a"}]
This command is special, because it allows you to format a message using JSON (JavaScript Object Notation) to allow colored text, scoreboard values, and selectors, and much more to be displayed in a chat message. There are a few similar commands, but /tellraw is the one you need.
You probably want to customize the text component to your liking, so here is a guide to creating the text components manually and here is a website that will allow you to create them automatically.
Player Count
To reproduce the /list command further using this method, you can use the /stats command, combined with a scoreboard. Create a new scoreboard of any name (PlayerCount), and set a fake player (#Count)'s score on that board to 0.
/scoreboard players set #Count PlayerCount 0
Now, it is time to use /stats. Within your command chain, before your /tellraw command, place a new command block with the following command:
/testfor @a
Then, stand on top of the command and run:
/stats block ~ ~-1 ~ set AffectedEntities #Count PlayerCount
Now, you will notice that each time your command chain runs, the player count score will be updated. You can edit your /tellraw text component to display this count. Here is an example:
/tellraw @a ["",{"text":"Welcome to the server! There are "},{"score":{"name":"#Count","objective":"PlayerCount"}},{"text":" players online now: "},{"selector":"@a"}]
Best Answer
You can use a repeating command block with testfor to detect if someone is holding the beetroot. If it passes true, it can detect if a command linked to stat.eat.beetroot (?) increases to one, reset it to 0, and execute whatever commands you would like. I'll post redstone and commands later when I can get on Minecraft.