El candado está en el portabultos.

Because Spanish normally uses estar for location.

• El candado está en el portabultos = the padlock is located on/in the rack.
• Ser is generally not used for where things are, except for events. For example:
  • La reunión es en Madrid = the meeting is in Madrid.

So for the physical location of an object like candado, está is the natural choice.

Portabultos is a noun meaning luggage rack, carrier, or rack for carrying things.

In Spain, it often refers to the rear rack on a bicycle, though it can also be used more generally depending on context.

It is a compound word:

• porta- = carries / holder for
• bultos = packages, loads, bulky items

So literally it is something like load-carrier.

Because Spanish en is broader than English in. It can correspond to in, on, or at, depending on context.

So:

• en el portabultos may be translated as on the luggage rack in natural English.

Spanish does not always switch to a separate preposition the way English does. Even when English prefers on, Spanish often still uses en for location.

Because both nouns are masculine singular, so they take the definite article el.

• el candado = the padlock
• el portabultos = the luggage rack

Spanish uses articles more regularly than English in many cases, and here both nouns need them.

For candado, the ending -o is a strong clue that it is masculine:

• el candado

For portabultos, the gender is something you usually learn with the word:

• el portabultos

Compound nouns like portabultos do not always have an obvious ending that tells you the gender, so the article is especially useful. If you learn the noun together with el, you will remember its gender more easily.

It is pronounced roughly:

por-ta-BUL-tos

The stress falls on bul.

A rough English-speaker guide to the whole sentence would be:

el kan-DA-do es-TA en el por-ta-BUL-tos

Also notice:

• está has a written accent because the stress falls on the last syllable: es-TÁ
• candado is stressed on da: can-DA-do

Usually candado means a padlock.

If you mean a built-in lock, Spanish may use other words, such as:

• cerradura = lock mechanism / lock
• bloqueo = locking system / blockage, depending on context

So candado suggests a separate lock, the kind you might attach to a bike or chain.

Yes, very often it stays the same in the plural:

• el portabultos = the luggage rack
• los portabultos = the luggage racks

This happens with some compound nouns ending in -s. The article usually tells you whether it is singular or plural.

This is the most neutral and natural order:

• El candado está en el portabultos

You can change the order, but it sounds more marked or emphatic:

• En el portabultos está el candado

That version might be used if you are contrasting locations or answering a question like Where is the padlock? The original sentence is the normal default order.