/testfor item in item frame help with code [duplicate]
Solution 1:
You had an obsolete } in the target selector. But probably you didn't have that in the game, because then your error message would have been different.
Your real issue: Since 1.9 you need quotes around the item ID:
/testfor @e[type=Item_Frame,x=-1285,y=72,z=788] {Item:{id:"minecraft:leather"}}
1.13 syntax:
/execute if entity @e[type=item_frame,x=-1285,y=72,z=788,nbt={Item:{id:"minecraft:leather"}} run <command>
You can also leave out run and the command if you just want a chat output.