I tried "Vy nevidis ty mysi" ( I know I haven't included the accents). Vy and Ty are both shown in the hints but only Ty is accepted as correct. Could someone please explain ? Thank you !
'Ty nevidíš' or 'Vy nevidíte'; both are valid here. The first one is singular informal, the second one sg formal and plural.
Would "ty nevidis ty mysi" (with accents) be right also?
Yes, sure, you can always add the personal pronoun if you want to stress it. Here it makes the question sound more incredulous (a bit like "Can't you really see those mice?")