Finally, choose Save (Ctrl+S), then Activate (Ctrl+F3) your class.
Your code should look like this:
CLASS zcl_invoice_retrieval DEFINITION
PUBLIC
FINAL
CREATE PUBLIC .
PUBLIC SECTION.
TYPES: ty_table_of_zso_invoice_item TYPE STANDARD TABLE OF zso_invoice_item WITH DEFAULT KEY.
METHODS get_items_from_db
RETURNING
VALUE(lt_result) type ty_table_of_zso_invoice_item.
PROTECTED SECTION.
PRIVATE SECTION.
ENDCLASS.
CLASS zcl_invoice_retrieval IMPLEMENTATION.
METHOD get_items_from_db.
SELECT
snwd_bpa~company_name,
snwd_so_inv_item~gross_amount,
snwd_so_inv_item~currency_code,
snwd_so_inv_head~payment_status
FROM
snwd_so_inv_item
JOIN snwd_so_inv_head ON snwd_so_inv_item~parent_key = snwd_so_inv_head~node_key
JOIN snwd_bpa ON snwd_so_inv_head~buyer_guid = snwd_bpa~node_key
INTO TABLE @lt_result
WHERE
snwd_so_inv_item~currency_code = 'USD'
ORDER BY
snwd_bpa~company_name.
LOOP AT lt_result ASSIGNING FIELD-SYMBOL(<entry>).
CASE <entry>-payment_status.
WHEN 'P'.
<entry>-payment_status = abap_true.
WHEN OTHERS.
<entry>-payment_status = abap_false.
ENDCASE.
ENDLOOP.
ENDMETHOD.
ENDCLASS.