Play with MT4 on demo. That way it won't cost you anything when you hit the wrong button.
There It's easy to get limit and stop orders confused. A Stop order is based on "If the price goes this far, I think it will keep going farther." and a Limit order is based on "If the price goes this far, I think it will reverse."
This means that a buy stop is set up above the current price and a buy limit is set up below the current price. If you try to do it the other way, you'll get an invalid price error. There's an easy way to avoid this. Right-click on the chart near the target price you have in mind and select "Trading". It will give you either buy limit and sell stop or a buy stop and sell limit as choices.
The other possibility is that your broker restricts how lose you can set limit and stop orders to the current price. If you are too close, you'll get an invalid price error. The rule about how close you can be varies a lot between brokerages, but is typically somewhere in the 3-15 pip range.