giving a score to an item with a Damaga value

Damage is stored as a short, rather than a normal integer. When testing for data tags you need to match the data exactly, including its type. Add s after the number to specify a short:

/scoreboard players set @e[type=Item] isYellowWool 1 {Item:{id:minecraft:wool,Damage:4s}}

The data tag '598f6a62-8ae2-4c54-b5c4-f26e4ba340f5' is probably your tag associated with your username on whatever server you were on. Now, I'm not sure, but I'd say that 'The dataTag does not match for 598f6a62-8ae2-4c54-b5c4-f26e4ba340f5' is a normal error. It just means that whatever data tag you set, may it be 'if any player is holding a stick' or, in your case yellow wool, then do something - which is set the scoreboard value - only, you aren't holding the wool, so the data tag does not match. Try putting your command into a repeat, always active command block then do '/scoreboard players list (Your Username) ' Then check your value for 'isYellowWool'. Then hold yellow wool, and check again. I think it was never broken but I'm not sure.