Create Custom Column using an IF Statement based on two columns

Craig_01 , Try using below formula for new calculated column

Difference = 
IF('Table'[Branch] = "York" && 'Table'[Status] = "Out",
    DATEDIFF('Table'[Despatch Date], 'Table'[Requested Date], DAY),
    DATEDIFF('Table'[Despatch Date], 'Table'[Promised Date], DAY)
)

Thanks for the reply from bhanu_gautam , please allow me to provide another insight: 
Hi  Craig_01 ,

Here are the steps you can follow：

1. Add Column – Custom Column.

2. Enter code.

if [Branch]="Your" and [Status] ="Out"
then Duration.Days([Despatch Date] - [Requested Date])
else Duration.Days([Despatch Date] - [Promised Date])

3. Result:

Best Regards,

Liu Yang

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