"couldn't reach the mailbox" message

This means your mailbox isn't reachable for the mail carrier to do the job , like you delete mail box without noticing while building, something blocks the way to the mailbox, this including your sim is interacting with the mailbox while the mail carrier is about to deliver. Or your mailbox is inside your fence and you lock the fence's door, which makes it impossible for the carrier to enter and deliver. As long as that message keeps showing up, it means something must be wrong about the progress of delivering. Once you see the mail carrier show up and do put something into the mailbox, that means it's fixed. And you can tell who the mail carriers are easily because of their unique outfit.
But still, sometimes I do get the message even though my mailbox is fine. If this happen, maybe it's the NPC who plays the mail carrier has issues like glitch. Try to delete that NPC to generate a new one. If still not working, it could narrow down to a bigger in-game problem. Try to repair the game to see if it helps. If still not, go to report a bug for mail carrier not doing their jobs.