I tried to install KnightOS on my TI 83+SE using TiLp and the installation failed. I then tried to reinstall TI OS using both Ti Connect and TiLP.

I got these errors:

TiLP:
The cable cannot be used.
Cause: the cable has not been initialized due to a previous/current error.

TI Connect
Cannot transfer the application to the connected TI device.
A certificate is required for a successful transfer. (8C02001D)

I assume that means that both softwares could not connect to the calculator. Any help?

Try the standard bootloader key sequence for putting the calculator in OS receive mode:
1) Remove 1 AAA battery
2) Hold the [DEL] key
3) Re-insert the AAA battery while continuing to hold [DEL]
4) Press and release [ON] while continuing to hold [DEL]
5) Release [DEL]. The calculator should say "Waiting for OS..." or equivalent.

Reading SAX last night, I believe Unicorn is already at step 5. If there's no OS, the missing certificate warning sounds pretty ominous.

When I do as you say, It just turns the calc off. Now, if I re-insert the battery then release DEL it gives me the waiting for OS screen. But, trying to install the OS results in the errors I posted above.

It gave me this error on calc when I first tried to install KnightOS:
ERROR!
Press any key to turn unit off. Then turn unit back on.

Except when I turn it back on it is waiting for OS again.

I believe that another user was having similar problems with a wiped certificate. The moral of the story seems to be that third-party OSes, especially those with code that could stand a bit of code review, should be approached with great care.

This is why I have yet to try any existing third-party OS that is not finished on an actual calculator. Even FiXOS despite the FX-9750GII only costing me $35.

I think I fixed it! I tried installing KnightOS using TI Connect this time and the install is at 50%. So using TiLP is buggy then.

EDIT: My conclusion is to refrain from using TiLP to change OSes, because it seem that TiLP was the problem for me.

Using TiLP 1.18 on Ubuntu, I've never experienced any problems computer-side, except when I tried to send GlassOS to my calc. But I'm pretty sure it was the .8xu's fault.

This is on windows 7.

I assumed so (TI-Connect only runs on Windows and Mac), my point was that TiLP's never given me problems. I also have used TiLP to send an OS successfully on Windows Vista, too.

IIRC I saw IRC logs somewhere (I forgot which calc site) where somebody else had issues sending an OS using TiLP a few days ago). Not sure if it was Unicorn but if it was somebody else then maybe it might be a TiLP bug and that you should report it to the team.

Yeah, I will admit that TiLP seems buggy, but what I do to avoid many issues is always refresh the directory list before doing anything like sending a program.