I cant detect the items inside the chest here is the command:
/testforblock 691 18 -317 chest 4 {Items:[{id:minecraft:diamond,Slot:0b,Count:2b}]}
here is the error dysplay the command block:
[20:11:07] El análisis de la dataTag falló: Expected '}' but got ':' at: {Items:[{id:minecraft:<--[HERE]
Unable to replicate, the command works fine for me. Given output (with chest filled with diamonds)
[11:47:44] The block at 65, 1, 2 had the data value of 3 (expected: 4)