Talk about what you found difficult during your training. It may have been memorizing medical terminology or studying for long hours at a time. Share with the interviewer why it was difficult. Remember, you're role in this interview is to talk about your strengths and use weaknesses to show your adaptability.
"I struggled with biology courses in high school. When I started my online training be a pharmacy tech I knew I would need extra help in my pharmacology course, so I went to tutoring and studied hard. I passed the PTCB exam with flying colors and was able to get a job soon afterwards."
"Calculating drip rates were challenging for me. I worked with a mentor during my training that helped me learn it a different way that was easier for me to understand. I can calculate drip rates with my eyes closed now."