The select="multi" attribute is not supported by Odoo's selection field in the search panel tag of a tree view. Only many2many fields can use the select attribute, which lets you choose several values from the field.
On the other side, the selection field is used to offer a drop-down selection of predetermined values. Multiple selections are not by default permitted.
Use a many2many field rather than a selection field if you need to make several selections available in the search panel for a field. The underlying data model and the associated views would need to be changed to accommodate this modification.