Output Placeholder, Max Length, Line Breaks, Initial Rows values for Fields in Sprout Forms
The problem is due you're trying to get a property from the FieldLayoutModel instead of FieldLayoutFieldModel please see the code bellow to fix the error.

{%- set layoutFields = tab.getFields() %}

    {% for layoutField in layoutFields -%}

        {%- set field     = layoutField.getField() -%}
        {{ field.settings.placeholder }}
    {% endfor %}