Kontantkassen med vekselpengene er låst.

Because Norwegian very often makes compound nouns by joining words together.

• kontant = cash
• kasse = box / till / cash box
• kontantkasse = cash box / cash till

Then -n is added to make it definite:

• en kontantkasse = a cash box
• kontantkassen = the cash box

This is very normal in Norwegian. Where English often uses two words, Norwegian often prefers one compound word.

In Norwegian, the definite article is usually attached to the end of the noun instead of standing in front of it.

So:

• en kasse = a box
• kassen = the box

And similarly:

• en kontantkasse = a cash box
• kontantkassen = the cash box

So the sentence does not need a separate word like English the, because that meaning is already built into kontantkassen.

med means with, so med vekselpengene means with the change or with the change money.

In the sentence, this phrase describes kontantkassen:

• Kontantkassen med vekselpengene = the cash box with the change

So the structure is:

• [subject] Kontantkassen med vekselpengene
• [verb] er
• [description] låst

It does not mean that the box is locked using the change. It means the cash box that contains or goes with the change is the one being talked about.

Vekselpenger means change in the money sense, like the coins and small notes used to give customers the right amount back.

Norwegian often uses penger as a plural form, even where English just says money. So:

• vekselpenger = change
• vekselpengene = the change

The definite ending -ne makes it specific. Here it refers to the particular change associated with that cash box.

Because Norwegian treats penger as a plural noun. This is one of those places where English and Norwegian do not match neatly.

Examples:

• Jeg har penger. = I have money.
• Hvor er pengene? = Where is the money?

So even though English uses the singular-looking word money, Norwegian uses penger / pengene.

That is why vekselpengene is completely natural.

It is the past participle of låse (to lock), but in this sentence it functions like an adjective meaning locked.

• å låse = to lock
• låst = locked

So:

• er låst = is locked

This is very similar to English. Compare:

• The door is closed
• Døra er lukket

In the same way:

• Kontantkassen ... er låst = The cash box ... is locked

Because Norwegian commonly uses være + past participle/adjective to describe a state.

Here, er låst means the box is in a locked state.

That is different from focusing on the action itself. For example:

• Kontantkassen er låst. = The cash box is locked.
  (state/result)
• Noen låste kontantkassen. = Someone locked the cash box.
  (action)

So er låst is the natural way to say that it is currently locked.

Usually it means is locked: it describes the current condition of the cash box.

Depending on context, English might sometimes translate a similar idea as has been locked, but the most direct meaning here is the present state:

• Kontantkassen med vekselpengene er låst. = The cash box with the change is locked.

If you wanted to clearly emphasize the action that happened earlier, Norwegian would usually phrase it differently.

This is standard Norwegian word order for a normal statement:

• Subject + verb + complement

Here:

• Kontantkassen med vekselpengene = subject
• er = verb
• låst = complement / description

So the sentence follows a very normal pattern: The cash box with the change + is + locked.

Yes, in many varieties of Bokmål, especially more informal or everyday style, -a is often used for the definite form of feminine nouns:

• ei/en kasse
• kassa = the box

So you may hear:

• Kontantkassa med vekselpengene er låst.

That is also natural Norwegian.
Kontantkassen is perfectly correct too, because many feminine nouns in Bokmål can also take the masculine-style definite ending -en.

Not necessarily. Depending on context, Norwegian might also use words like:

• kassa = the till / the cash register area / the checkout
• pengekasse = money box
• kassaskrin = cash box / cash case

But kontantkasse specifically suggests a box or cash container holding cash, often including change. So in this sentence it is a very reasonable and precise choice.