Hi, can anyone explain error: Keyword and non-keyword arguments must not be mixed,

https://docs.appian.com/suite/help/20.4/Expressions.html#passing-arguments

You may not mix both styles.

Yes this error will occur if you are missing a keyword such as below.  If you are debugging a large form, try commenting out sections at a time until you narrow it down.  Feel free to post some code for review here as well. 

a!localVariables(
  local!data,
  a!textField(
    label: "Text Field",
    local!data /* <- missing 'value' keyword */
  )
)