Simplistically, a production tax credit is when every time you produce a unit of something, you get a payment. For a kilowatt of solar power from a solar panel, the United States government will give you a payment of, say, seven cents for a kilowatt as it's produced.
An investment tax credit means that you first have to make the investment—put your money up front—and then get your money back after, by offsetting the taxes you have to pay on your profitability. It's a more complicated way of thinking about it. It doesn't necessarily mean that it's less financially lucrative, but it's more complicated to model, especially when you have the layering, chaining and clashing policies all across the energy system and, on top of that, the uncertainty and volatility in the carbon markets, which are opaque.