There was a time when you could put coins into a slot machine, but that time is largely gone. However, while you may not be able to feed a machine with coins anymore, you can still use banknotes. There should be a slot that will accept notes in various denominations. This slot usually takes payout tickets you may already have too. So, if you played a slot machine before and received a payout ticket to cover anything you won from it, you can use that ticket to feed the new machine.
However, you may still see some slot machines that accept coins rather than banknotes. They tend to crop up in casinos that see a lot of traffic, as it is often faster and more convenient to allow people to play with coins. It does depend on the game though. If you spot a slot machine with large denominations possible on it, you will likely need to get a ticket before you can play. That ticket will then be inserted in the machine and you can get started.
You can see you cannot ever be 100% certain that a slot machine will accept cash. Cash might mean coins for some, banknotes for others. And you’ve got tickets to play even more. Check in advance when visiting a casino and see which machines use which deposit methods. You will then know what you need to play with.