It does - of course - have a perfectly valid header. One of the reasons this sometimes happens with WP plugins (any plugin) is copying it in twice - or a glitch occurring while it was being copied in;. This can cause WordPress to add a number to the end of the plugin folder name.

If possible can you check your plugins folder (wp-content/plugins) and make sure that the simple-press folder created is, in fact, named 'simple-press' and also that there are not two folders with the same plugin inside as there must be only the one.