Replace the semi-colons with a comma, so that the formula for finish is "Week " & CStr(1+datediff("w","1/1/20" & mid(Year([Finish]),3,2);[Finish]))

explanation.

mid(Year([Finish]),3,2) = extracts 2 characters from the year of the finish date starting at the 3rd character. E.g. for "04-Jun-21", year is 2021 and the formula extracts 21.

The 21 from above is then joined to "1/1/20" to form "1/1/2021".