Players cannot have sleeves of any kind, including turf tape, paired with an untucked undershirt. The error icon pops up saying you “cannot equip left/ right arm with current undershirt…” This makes no sense how the two are related. However, players may already have sleeves and an untucked undershirt. Caleb Williams is my example here. He has sleeves and an untucked undershirt. If you change he sleeves in any way you’ll lose the undershirt. This is mostly a nuisance with drafted players in franchise.