So now we know that your theme breaks Toolset Forms, and likely this requires a compatibility process that involves that themes's author, do you agree?
If the Theme breaks the Form, we should ask the Theme Authors to cooperate with us an fix it, once we know why it's happening.
You use a Custom theme, we cannot support or fix such themes.
I suggest to - if your theme breaks Forms, using another.
Related to the issue:
You want to create many Products with Forms and choose each new product one parent of a certain type.
You have one relationship on this site:
accommodations-product
It connects many products with one accommodation each.
As I show here, https://toolset.com/forums/topic/product-as-a-child-of-post-type/#post-1070011, if you now (I did that for you here hidden link) save a new Form to add products, the slug to add parents is not what you had in the first form, but instead as elaborated on the first reply shared above.
When I now insert this form to a page, and submit it, a new product should be created with a set parent.
When I tried to insert the form and create a page the website went down with a E 500.
I assume you have been working on some updates or similar, however, the Page was probably not saved.
You can try the new form with [cred_form form="create-new-product-and-assign-a-parent"], on any page, it should work.
Note that I cannot fix issues for you, instead, I can instruct you how to.
I have followed my own instructions when I did the steps above and was about to test it on your site but it went down with the E500 mentioned.
I suggest, l to try that form.
If it doesn't work, please let me know or provide a copy of the site, if the issue persist after all other plugins and themes are removed.
I as well tested this again locally, it works fine.