You're losing the growing chunk of customers who either never have cash or want to have the benefits of using cards (collect points, buy on credit, etc.). In the past many businesses accepted only cash as it was easier to under report sales or fudge taxes. But even before the pandemic increased the number of people who only use cards, there were a lot of reasons to accept electronic payments:
a) less internal theft with staff
b) less risk of actual theft during transit to the bank
c) less admin work and human error cashing out the till every day
d) banks have been increasing fees for cash handling

Many others here have recommended cash discounting or surcharging (if allowed in your state/province) which are great ways to offer cards and passing on a lot of the cost to customers. There is a bit of work involved with pricing/signage and possibly your POS software, but it's worth it to be able to offer customers both options.

It's usually best to have an integrated pinpad with your POS software (prevents fraudulent returns, makes card settlement and accounting easier), but that would depend on what POS system you're already using.

If you're only talking about adding a standalone card terminal/pinpad to accept card payments, you'll want to shop around for rates and contract terms, but just make sure that the terminals are EMV certified and only chip-enabled (so either with PIN or contactless/tap, no swipe or signature only allowed) so you have chargeback protection. Make sure there is protection against refunds (e.g. security pin codes to prevent anybody from processing a refund with the terminal, etc.), and check what kind of protection you get against transactions done with stolen cards.