Problems with using checkbox with Thymeleaf

as I wrote the title, I'm having problems with using checkbox with Thymleaf.

My Form has a List of object "AssignFlag" with name "assignFlagList".

List<AssignFlagForm> assignFlagList;

object AssignFlagForm has 3 fields:

    String codeValue;

    String codeKey;

    public boolean isChecked;

I want to create checkboxes for each AssignFlagForm, and post "true" value for each AssignFlag that is checked on the screen.

my thymleaf template goes:

                 <span th:each="assignFlag, stat : *{assignFlagList}">
                            <input type="checkbox" th:field="*{assignFlagList[__${stat.index}__].isChecked}"
                                   th:value="*{'TRUE'}" style="margin-right: 0;" />
                            <label th:for="${'assignFlagList' + stat.index + '.isChecked1'}" th:text="${assignFlag.codeKey}" style="font-weight: normal;">
                            </label>
                        </span>

and I wrote this referring to the official document of Thymleaf https://www.thymeleaf.org/doc/tutorials/2.1/thymeleafspring.html#checkbox-fields

but it doesn't work and I don't know why. Error code goes Error during execution of processor 'org.thymeleaf.spring5.processor.SpringInputCheckboxFieldTagProcessor' but I still can't figure out.

Could somebody please help me?

Thank you in advance.