Learn Spanish (Spain): Ella dice que el tique está en la caja fuerte. - She says that the receipt is in the safe.

The verb decir can be used to report information or to give a command. Here, she is simply reporting a fact, so we use the indicative está. If she were giving an order (for example, telling someone to make sure the receipt is in the safe), it would act as a command and trigger the subjunctive: Ella dice que el tique esté en la caja fuerte.

Is tique just the English word ticket?

It is a loanword adapted from English, but its meaning is narrower here. In Spain, un tique is specifically a store receipt from a cash register. If you are talking about a ticket for a train or a concert, you would use un billete or una entrada instead.

How do you make the compound noun caja fuerte plural?

Because caja fuerte (safe) is made up of a noun (caja) and an adjective (fuerte), both words must take the plural form. You would say las cajas fuertes.

Could I use es instead of está to say where the receipt is?

No, you must use está. A common trap is thinking that permanent locations use ser and temporary ones use estar, but the physical location of objects (like el tique) always takes estar regardless of time. You only use ser for location when talking about where an event takes place.