Why can’t I use heavy armor??

Hi! So I’m currently playing a wizard/fighter multiclass (6 levels in wizard, 1 in fighter) and I’ve been loving it. Especially love the fact that I can have him use cool armor now with the proficiencies he gets from fighter, he currently has on +1 breastplate. But, I just picked up some plate armor and I really like how it looks on him but the game is saying I’m not proficient with heavy armor. I don’t exactly understand why? Because I thought fighters get proficiency in all armor, is there something I’m missing here?? I’m so confused