Sprout Forms: Populate checkbox field options with entry data
The best way to debug a form field using custom HTML is to output the field you want via the default displayForm tag and copy the code from there as a starting point:

{# Output your checkboxes field here and look at the code generated #}
{{ craft.sproutForms.displayForm('yourFormWithACheckbox') }}


Test with the default code that is output as your custom HTML and if that works, adjust from there.

In general, we recommend using custom HTML with caution as it can increase the complexity of the code on the front-end a great deal.