Urban Air Mobility (UAM) is a transport system enabling the movement of people and goods within urban areas using electric vertical take-off and landing (eVTOL) aircraft. Nonetheless, this concept remains an emerging technology with various challenges that can hinder its implementation. This research introduces a Multi-Criteria Decision-Making (MCDM) framework to prioritize barriers to UAM implementation in Germany and the USA. We identify 26 barriers across technological, economic, social, environmental, and operational aspects through a comprehensive literature review and expert interviews. Using the Fuzzy Best-Worst Method (FBWM), we determine the weight of each barrier based on input from industry and academic experts in Germany and the USA. Our findings reveal that economic aspects pose the greatest challenge in Germany, followed by social, operational, technological, and environmental aspects. In the USA, operational aspects are the most significant, followed by technological, economic, social, and environmental aspects. The operational aspect shows the largest difference between the two countries, while the environmental aspect shows the least. Globally, the top three barriers in Germany are price affordability, investment uncertainty, and user acceptance concerns. In the USA, the top three barriers are airspace utilization challenges, remote/autonomous operations, and system safety and cybersecurity issues, which rank tenth, twelfth, and sixteenth, respectively, in Germany. We also discuss the potential implications of our findings, offering strategies to effectively address high-priority barriers.