Multiple values in SQL for Execute Query to excel

FileNames = String.Join(“,”,TxtFileName) → Write Line String.Join(“,”,TxtFileName).Split(“_“c).last → Execute Query “select street_no, phone_no from data where phone_no in ('” + String.Join(”‘,’”.TxtFileName) + “')”

With the error message TxtFileName is not a member of ‘String’.

FileName i declared it as String
TxtFileName declared as System.Collections.Generic.List<System.String>

did I put the wrong variable?