I need help with using fresh lemon juice or if I can use lemon juice concentrate.

When I joined Food52 in 2009, one of the biggest things I learned was what a difference the quality of ingredients makes. Especially with a simple recipe where the ingredient of concern is the main flavor. As 702551 said, there is no pre-squeezed citrus that comes close to fresh. You can make this really delicious (as opposed to okay) by squeezing the juice yourself. A hand juicer like this one https://food52.com/shop/products/4848-citrus-juicer or even a wooden lemon reamer (you can strain out the seeds) like this https://www.amazon.com/OXO-Good-Grips-Wooden-Reamer/dp/B0007VO0DA will make the job easier. Good luck! It’s a great pie.

Never in my life have I had any pre-squeezed citrus juice that was good as freshly squeezed whether it was frozen concentrate, jarred, or canned.

Heck, even day-old refrigerated fresh squeezed juice isn't as good as the stuff squeezed a few minutes ago: this is particularly noticeable with grapefruit juice.

There's a reason why expensive juicing machines (both consumer and commercial) show up in home kitchens, cafes, restaurants, and hotels. Same with bartenders squeezing citrus right there into a cocktail glass.

Of course, you can still use pre-squeezed juice in this recipe and it will come out. You'll be gaining convenience at the expense of taste.

It's really up to you to decide whether or not that trade off is worth based on your taste buds and those who will be joining you at your dinner table.

Personally I would spend a few extra minutes juicing the citrus myself but I won't be sitting at your table.

Entirely your call.

Best of luck.