1. Check spam and other folders
Search your email (including spam, junk and promotions) for kimsim or support@kimsim.co. The one you want is called "Your … eSIM is ready to install".
2. You don't need the email
Your eSIM is also in My eSIMs. Sign in with the email you paid with to install it from there.
Paid with Apple Pay?
The email may have gone to an Apple Hide My Email address, which forwards to your Apple ID email. More.
Typo in your email?
The install screen right after paying still works. Then email support@kimsim.co from your correct address with your order number, and a person will move the eSIM to you. Or use the contact form.