Smartsheets just recently released a new multiselect field type. This field type does not yet update from the EnumList field type in Appsheet. Doing so would unlock this functionality fully.
The work around is simple
-Smartsheet => create a single select field
-Smartsheet => allow other entries apart from the dropdown list
-AppSheet => (re)generate column structure to populate the Enum field
-AppSheet => ensure that the column type is set to Enum List
This will create a new entry in the smartsheet cell consisting of a list of all the selected options separated by a comma. But it makes for difficult reporting and makes automatic counting of selections nearly impossible.