THE EU VISA THICKET: when UK business people restart travelling to the EU they face a new world. If you're charging a fee, and sometimes even if you're not, you will need a work permit - and each of the EU27 has its own rules.... https://www.ft.com/content/bd9f9910-aa3f-4dbb-a4ca-ed8bb1f65349#comments-anchor
Even for unpaid work, if you provide any sort of service, Denmark, Croatia and Cyprus will make you apply for a work permit. If you or your company are being paid, you will almost always need a work permit. Some may shrug....