This and this post on main meta (and the comments on those posts) seem to say that the GDPR doesn't act on the level of treating a question or answer as 'personal data'. So, being unable to delete your question because it doesn't meet the criteria to allow you to do so does not seem to be a violation of the GDPR.
Usually, the way to go about such things is to request dissociation of the post from your main account:
If you would like to have your name dissociated from one of your posts, you can request we do so by clicking on the 'contact us' link at the bottom of the page. Your username will no longer be linked, the username will be shown as "anon", and you will no longer earn any reputation from the post. All reputation you had previously gained/lost from the post will also be reversed, and badges you earned from the post may be revoked.
Looking at your account though, it seems likely such a request may end up being declined, as this is your only post on IPS and this post seems to suggest that deleting your account on IPS might be favorable if you want to be dissociated of 100% of your posts on this site:
If you request that a large percentage of your posts be disassociated from your account, we might request that you simply request account deletion (which attributes your posts to an anonymous user), and start over.
In this case, deletion may be favorable because otherwise the question is still traceable to your account through this meta post.