Hi Atmospherium!
Your problem is here:
forEach
doesn’t return a value; it just does some operation on every value in the collection.
I think what you’re trying to do, is for the current message author, to get their inventory like {"master-crate": 1, "hyper-crate": 48}
and perhaps render that down to a string? Did you want to show 0 for the crates that they don’t have, or just exclude them?
I see that addField takes a value that has to be ‘StringResolvable’… maybe an object/array will work there, maybe not. If you are trying to render the whole thing down to a string then you can use reduce
.
Here are some examples to try. I have restructured your code so you can log the object out and inspect the values.
Just show the crates they have
const inventoryArray = File[Message.author.id];
// Should be {"master-crate": 1, "hyper-crate": 48}
console.log(inventoryArray);
Message.channel.send (
new MessageEmbed ()
.setColor ('GOLD')
.setTitle (`${Message.author.username}'s inventory:`)
.addField ('Inventory:', inventoryArray, true)
)
Create an inventory array that includes zeroes
Use forEach to push each key-value into the object
let inventoryObject = {};
Crates.forEach(Crate => inventoryObject[Crate] = File[Message.author.id][Crate] || 0);
// Should be {'daily-crate' : 0, 'beginner-crate' : 0, 'master-crate' : 1, 'hyper-crate' : 48};
console.log(inventoryObject);
Message.channel.send (
new MessageEmbed ()
.setColor ('GOLD')
.setTitle (`${Message.author.username}'s inventory:`)
.addField ('Inventory:', inventoryObject, true)
)
If addField requires a string
This is where you can do your formatting with \n
newlines as in your original.
let inventoryString = '';
Crates.forEach(Crate => inventoryString += Crate + ' : ' + (File[Message.author.id][Crate] || 0) + '\r\n' );
// Should be a string like:
// "daily-crate : 0
// beginner-crate : 0
// master-crate : 1
// hyper-crate : 48"
console.log(inventoryString);
Message.channel.send (
new MessageEmbed ()
.setColor ('GOLD')
.setTitle (`${Message.author.username}'s inventory:`)
.addField ('Inventory:', inventoryString, true)
)
Hope that gives you some options to play around with. All code untested 
Have fun!!
Check out:
Array.map - MDN
Array.reduce- MDN