I purchased iPhone 13 but the charger is missing

It's gone.

About Apple USB power adapters - Apple Support

As part of our efforts to reach our environmental goals, iPhone models no longer include a power adapter in the box. iPhone continues to work with existing power adapters and USB-A to Lightning cables, such as those included in prior models. iPhone models now include in the box a newer USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports that you might already have. You can use either a USB-A to Lightning cable or the newer USB-C to Lightning cable with your iPhone.

Apple along with other manufacturers did away with supplying chargers about 18 months ago when 90% of the population already had some sort of USB charger. This is for earth sustainability reasons and only Brazil has objected to the change, every other country has lauded the decision

Choices

• Use your old iphone charger with the old lightning cable
• Use your old iPhone charger or any other USB-A source witrh the new cable and a USB-A male to USB-C female adapter
• Buy the proper 20 Watt charger from Apple.com for £20 or £20 and enjoy fast charging