It is your responsibility to check your passport validity.
Serbia - Your passport must have an ‘expiry date’ at least 90 days after the date you plan to leave Serbia. You can visit Serbia without a visa for up to 90 days within a 6-month period, for business or tourism. Make sure you get your passport stamped.
Bulgaria - Bulgaria follows Schengen area rules. Your passport must have a ‘date of issue’ less than 10 years before the date you arrive and an ‘expiry date’ at least 3 months after the date you plan to leave the Schengen area. You can travel without a visa to the Schengen area, which includes Bulgaria, for up to 90 days in any 180-day period. Make sure you get your passport stamped on entry and exit.
Kosovo - To enter Kosovo, your passport must be undamaged and valid for the duration of your stay. You do not need an additional period of validity beyond this. You can visit Kosovo without a visa for up to 90 days, for business or tourism.
North Macedonia - To enter North Macedonia, your passport must have an ‘expiry date’ at least 90 days after the date you arrive. You can visit North Macedonia for up to 3 months without a visa.
We strongly advise you to check with your destinations' Tourist Board or Embassy for the most up-to-date guidelines regarding required passport validity and/or the FCDO https://www.gov.uk/foreign-travel-advice