New to working on cars.. did I mess up? Lifter replacement 4.7

With the rocker arms and lash adjusters out, you wouldn't have any compression because the valves are remaining closed. As you installed them (if the spark plugs were still in), you should have encountered some compression, but if the lash adjusters were new and didn't have much fluid inside, they may not have been offering much resistance.

Before you attempt to start, strongly recommend you remove the fuel pump relay, make sure your battery has a good charge, and crank the engine over for 10-15 seconds at a time, with a 2 minute delay for the starter to cool, and do this two or three times. This will allow the oil pump to fill lubrication passages and pressurize the lash adjusters. (You can also listen for any unusual noises in the valve train or top of the engine.) Then replace the fuel pump relay, turn the key on for 5 seconds, off for a couple of seconds, then on for 5 seconds, off for a couple of seconds, then start. This latter sequence to allow the fuel pump to pressurize the fuel rail.