you will get an error message

Select Customer.CustomerID from salesorder

table Customer has not been defined in the subquery.

The subquery should read

Select salesorder.CustomerID from salesorder

or

Select CustomerID from salesorder

There is no need to do a subquery, you could (and should) use a join

Select distinct

C.CustomerID

FROM

Customer C

INNER JOIN

salesorder S on S.customerID = C.CustomerID