So, as error message says. The element you have selected is SPAN. But, when you use Select class, it expects your element to be Select.
Observe your element and figure out what is the tagname of your element.
There are three possibilities I see here,
1. You might have chosen wrong element identification method. The method may be returning multiple elements and an element with span may be the first one.
2. When you are trying to select your dropdown, someother element is overriding it, and you are selecting it by mistake.
3. Dropdown looks like dorpdown, but it may be not. I mean, it can be a dropdown generated using JavaScript. So, it will look the same, but will behave different. In that case you can try using SendKeys, instead of Select class.