basiclly theres a few errors that may pop-up while filling your LSPD application
if it's this one (as it's the most common error message:
you need to enter at least 600 characters at your answers before you can submit it, just add more words untill it send it.
if you get an different one, as there's alot of different errors messages
just scroll around your application untill you notice something like this:
and then just fix whatever it ask you to fix, at that specific field.
if you still don't know what to do, post here an picture of the error over here for more assist.