Please avoid posting these sort of misleading questions.
select CategoryID, Quantity, Price
from Customer c, SalesOrderDetail sod
where sod.clientid in (select customerid where pricingplan='X')
works even without customerid in any of the table but not the intended result.
If any table has customerid. eg: customer table has customerid then alias name should be there otherwise gives ambigous column
select CategoryID, Quantity, Price
from Customer c, SalesOrderDetail sod
where sod.clientid in (select c.customerid where pricingplan='X')