After I had not been able to take money from any of the 4 ATMs at the airport, I finally found one in Ladyville. When that money finished I was in San Pedro. I went to an ATM on the main square and oh no! Again no money and the same error. I spoke to locals and their transactions were fine. At the hostel a girl who worked there told me that foreign bank cards have chips that the ATMs from Belize can’t read. She advised me to go to Scotiabank, next to San Pedro’s Airport, but also that did not work. Eventually I met a student who was also trying to use the ATM and he brought me to the only ATM machine I had not tried (nor seen) in San Pedro of the Hermitage bank! It worked! Remember that you can only take maximum 250 USD out every time. You can do it 2 times maximum after each other and within a day, so that is 500 USD maximum per day.
Check with your bank if they cover Belize and ask about the ATMs.