An Armstrong that will not start after a build is nearly always a solder joint that has not quite taken rather than a faulty board, and it is usually a quick fix with the iron you already have.
Go along the pins and check three things:
No solder bridging two neighbouring pins.
No loose beads of solder left sitting on the board.
Solder covering the bottom of each pin where it comes through the board, not just sitting on the pad.
The picture below shows good and bad joints side by side. The ones marked OK are what you are aiming for.
Reflow anything that does not match, then switch the Armstrong on again and see whether the display comes up.
If it still will not start, email us at [email protected] with a clear photo of both sides of the board, and your order number if you have it, and we will work out what is going on from there.