"I just have two shirts."
Translation:Yo apenas tengo dos camisas.
When you tap "just" you get a list of Spanish words that mean "just," and none of them are "apenas." I looked up "just" on two other language programs and neither of them define "apenas" as "just" either. They do, however, define it as "barely." English speakers are unlikely to use "just" this way or in this context. But just the fact that when you tap "just" Duo gives you a dropdown list of Spanish words that mean "just" and none of the choices are "apenas" tells you what you need to know. Either the English sentence is wrong in Spanish or the Spanish sentence is wrong in English. In any case, not going to learn Spanish from badly designed questions like this one.
That sentence is wrong in Spanish. You use "unico/a" as an adjective, it modifies the noun and means is the only one. So if you have two, it isn't unica. You might use the plural (unicas) but it would be before "camisas" since that's the noun the adjective qualifies, as in "tengo dos unicas camisas", and still isn't natural nor it means "just two shirts"; it would translate "I just/only have these two shirts"