Nut brittle -what went wrong?

You didn't heat it enough. "Turning light amber" means that you want caramel. What you did instead was to evaporate water from a sugar syrup, without reaching caramelization temperature. The crystals are not unusual (at some point you get a supersaturated syrup, after enough water has evaporated), but it is better if they don't happen, so don't give them occasion (use a clean pot, don't stir, etc. I suspect the oiling of the pan is meant to reduce nucleation sites).

The recipe is a bit strange anyway, using way too much water. Maybe you can try another one, if this has negative comments. The regular recipes will use some more conventional nut instead of pumpkin seeds, but this doesn't affect the physics of making the caramel. And do yourself a favor and use a candy thermometer. There are experienced cooks who can make sugar candy without one, but if you are not one of them, you'll save yourself lots of errors (and probably enough money in expensive ingredients for the thermometer to pay for itself).

Here is another recipe for brittle. While I haven't tried to make it, I strongly suspect that they know better what they are doing - not at least because they give you the exact temperatures you are aiming for. This is a site with lots of useful tips for candy making, and if you really want to try this without a thermometer, use their chart to learn about the different stages of sugar syrup and caramel.

And choose your recipes carefully. Candymaking (both sugar and chocolate) is even more exact than baking. Directions like "put back on medium for some time" are practically useless. In order to successfully cook a candy recipe, you have to know if not the exact temperature, then at least the desired sugar syrup stage. Maybe somebody who has made lots of brittles could work with a recipe like the foodnetwork one, noticing the problems on the fly and making the appropriate corrections towards the ideal brittle mass. If you don't know what the ideal brittle mass looks like before you start, it is better not to try it that way and go for an exact recipe.

"...until light amber" while not boiling hard indeed sounds like the procedure for invert syrup, but in the recipe you link there is a crucial step/ingredient missing (if invert syrup is indeed wanted): ACID. A lemon or two worth of juice should go into the syrup at the beginning, and the syrup should be simmered not boiled, and at the end be partially or wholly neutralized with baking soda - foams like rabies so be careful! Also it is better to underneutralize than overneutralize - sour syrup is tasty and keeps great, alkaline syrup on the other hand... if you store some, be sure to put a "dn??S" label on :)