Because it is a an arithmetic operation (you are trying to divide a number by 60)

Therefore you need an arithmetic expansion ( see: Shell command language: arithmetic expansions )
so you can assign the result of the arithmetic expression to a variable (assignment means variable=value)

If you do not do that, in your example AnzahlUeberstunden=$AnzahlUeberstunden/60 what happens is that you assign the value of AnzahlUeberstunden concatenated with the text string /60, so the result becomes 2040/60.

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My second suggestion is called an arithmetic expression (which can be used in shells like ksh93, bash, zsh) in which variables can be modified without an explicit variable assignment.