From in app chat:
Unfortunately, many of our customers have had difficulties using their Monzo card in Bulgaria 
We think that this is partly due to the way some of the Bulgarian card networks process pre-paid card data.
Monzo cards will not work at any merchants where Unicredit Bulbank (Bulgarian United Bank) or Raiffeisen Bank terminals are used.
Most shops have their tills connected to either of those two banks, and users receive a “Card number invalid” error message.
However, the Monzo card does work at about half of the sales terminals in Sofia.
Customers have found that the following ATMs are less likely to work with Monzo 
UniCredit Bulbank
Reiffeisen Bank
Investbank
Central Cooperative Bank
Société Generale Expressbank
On the plus side, customers have said that these ATMs are more likely to be successful 
PostBank
Banka DSK
First Investment Bank (FI Bank)
We’re working hard on rolling out our current accounts, after which using Monzo internationally will be even easier and a lot smoother 