I’m woking my way through the babypips school trying to set up a pending order on my demo account and Oanda is saying I can’t place the order because the, “Market is closed.” Am I correct to assume I should be able to set up a pending order even though the market is closed?
There’s some logic to it - you can’t open or close a bet when the market is closed either. and in effect a “Pending order” is simply you trying to “open a bet” but at a price other than “current market” - People could think about “pending orders” more - they can be a useful entry technique 
You are unable to book orders when the market is closed. There is a pre auction market before the open which allows market makers to assume and opening price.
The Canadian markets on the futures actually have a pre open auction available to anyone. I really like this as it makes for a fairer market.
I used to scale orders up and down for about 10 prices. And would usually be filled 5 up and then 5 down with a tick spread making 13,12,11,10 and 9 ticks prfit within the first 10 seconds of open. This was great and i used to hit it with 5 lots which in the CAD futures was about 30£ per tick. So not a bad morning.
But to answer your question, majority of markets, there is no ay for you to enter an order before it is open.
I guess that makes sense. On a related note, can you trade any currency pair as long as the forex market is open, or does a specific session have to be open based on the pair you’re trading? i.e., Can I trade anything I want from 6PM EDT Sunday through 5PM EDT Friday?
Should I consider what markets are open when making trades or setting up pending orders? i.e., should I be concerned about gaps if I trade the EUR/USD at a certain time of the day? FYI, I plan on starting with swing trading.