Try posting the post again first. Since the Pur Social service works with the social networks API, there are times when the social network itself does not work correctly.
If the post is not published again, refresh the page, check if you have lost access to your account ("Project Settings" → "Social Accounts"). If access is lost, click on “Renew access” next to your account. This will update access and you can try posting again.
You can hover over the account that failed the publication to view the error.
Sometimes it happens that Facebook does not skip publications with shortened links https://vk.cc. In this case, you need to remove this link from the post.