If the post you want to delete is the first post of a new thread, you can not delete it. Reason, it deletes the very reason for the thread and makes all things following nonsensical oftentimes.
If the post is not the start of a thread then there is some other issue and we need to figure out the technical reason behind the failure. Try accessing the site another way App <> Browser or changing Browser, refreshing page. If none of those work let us know what device and means of viewing the site you are using.
Try PMing the issue with links to the Lead Moderator, Laura Knotek to see what she can get done for you...