Some thoughts in random order:
You probably shouldn't have put any AF in your FW tank. Next spring you'll need to fill and drain a number of times to get all the AF out (I hope you've bypassed your water heater!).
Dumb question - Did you realign the valve back to the FW tank before you ran the pump again?
Some pumps are connected to the side of the FW tank rather than the bottom, so you need quite a bit of liquid in them to cover the connection and allow the pump to pump without sucking air. Six gallons may not be enough if your tank is like that.
If you line the valve back up to the AF connection, and attach the hose to it and if you can get some AF into the hose by holding it up and pouring the AF into the end of it (as opposed to just sticking the end of the hose down into the jug) you may be able to "force prime" the pump. Once it's primed, you can go back to sticking the hose in the jug.
Some units have TWO valves, not one. If you have 2 and didn't close the other, you'll suck air. See attached generic plumbing pic that shows several suction valving options.
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