CORRECTED:

Given the short notice in relation to this seminar, the press release was not clear. Any public press release having the mission name must be cleared at least five days before the event.

It’s an odd message, to release this by a mission yet they did not clear it. It appears you are violating the same rule you are giving to others. That’s not a grammar problem, it’s a content issue. If this is from “the mission” I would just leave the first line out- at least from what I can tell here.

“In regards to this seminar, due to the short notice, this press release was not cleared by mission (control?). In the future, any press release to be issued to the public bearing the mission name must be cleared at least five days before the event in question.”

Given the short notice, this press release was not cleared by the mission. Any press release to be issued to the public using the mission name must be cleared at least five (5) days before the event.

‘The’ instead of ‘this’ sounds better.

Also, the ‘5’ should be spelled out. (five)

I’m not sure though so don’t take my word on it! 😛