When I tried to insert a Mexican number, I am unable to input the international code for the number. For example,+52 1 8133233233 does not work but without the [1], +52 8133233233 works. Why does this happen?

This limitation exists because of Mexico's new policy.

You can submit the phone number without [1] and Facebook will automatically add back [1] later to your number.

For your reference: "As of August 3, 2019, phone numbers in Mexico have ten digits. The new national number was formed by appending the area codes to the local number [1]. All phone numbers should be dialed in a ten-digit format, even for local calls."