Check this first
Make sure you entered the number with the right country code. Some countries need the leading 0 removed.
If the app tries to verify by voice call instead of SMS, it can't be received. Ask for a text instead.
Wait or switch?
Most codes arrive within a few minutes. If nothing comes after 2 or 3 minutes, request the code once more in the app.
Still nothing? Cancel the number in your dashboard and get one from another country. Canceling returns the price right away.
Automatic refund
Even if you don't cancel, the price returns to your balance after 20 minutes. The number shows as "Refunded" in My numbers.
If the same app keeps rejecting numbers from several countries, contact support with the app name and order number.