First, rule out the simple causes
Confirm you copied the full address, including everything after the @, and that you pasted it into the site without a trailing space. One wrong character means the mail is delivered to an inbox that does not exist and you never see it.
Check whether a queue is showing on this page. New inboxes are rate-limited so the service stays fast; until your address actually appears, there is no mailbox to receive anything. Keep the tab open until the address is live, then submit the form.
Most verification codes arrive within a few seconds, but some senders take a minute. Leave this tab open and watch the inbox rather than refreshing away from it — the messages appear here automatically.
The sender may be blocking disposable domains
The most common reason a legitimate code never arrives is that the site refuses to send to known temp mail domains. Banks, large marketplaces, and some social networks maintain block lists of disposable providers. When that happens the mail is rejected on their side, so nothing will ever reach any temporary inbox.
If a form accepts the address but no mail comes, that is the likely cause. Trying a fresh inbox on the same domain will usually fail the same way. For those sites you generally need a real, long-lived mailbox.
What to try next
Generate a new address from this page and submit the form again — occasionally the first attempt was sent before the mailbox was ready. There is no spam folder here, so if it is not in the inbox above, it was not delivered.
Remember the timer: after five minutes the address and any mail are deleted permanently and cannot be recovered. If the code arrives late and the inbox has already expired, start over with a new address before you request the code.