My cookies were pasted to the parchment paper and very difficult to "peel" it off. Any suggestions

I make these cookies a lot, and that has certainly happened to me. Usually it's because the cookies are under-baked or over-cooled. You want to bake them till they're dark golden brown as in the photo. I find that the cookies peel off the parchment more easily about 3-5 minutes after taking them out of the oven. I also get a better result if I use twice as many almond slices as the recipe calls for. I've also tried using 2 egg whites instead of a whole egg as in this recipe https://www.splendidtable.org/story/2013/12/03/almond-and-orange-florentines, and they're a little drier and easier to remove.

See also "10 tricks glfor foolproof meringues" at The Mercury News.

You have several options to try.

The first would be to grease the parchment paper either with butter/oil or with a baking spray.

The second would be to try a different brand of parchment paper. They aren't identical.

The third would be to use a silicone baking mat. This was mentioned by at least one recipe commenter as successful.

Fourth, another commenter mentioned the possibility of baking these in silicone muffin cups.

I did not read all of the comments but there may be more useful testimony.

Best of luck.