For troubleshooting purposes only, I'm looking at just the vlookup without the iferror.
VLOOKUP(D14,'\\SERVER\data\Company Standard\TIME SHEET\Jobs\[Jobs.xlsx]2016'!B1:C300,2,FALSE)

Do either of these work
VLOOKUP(D14+0,'\\SERVER\data\Company Standard\TIME SHEET\Jobs\[Jobs.xlsx]2016'!B1:C300,2,FALSE)
VLOOKUP(D14&"",'\\SERVER\data\Company Standard\TIME SHEET\Jobs\[Jobs.xlsx]2016'!B1:C300,2,FALSE)