Q.spirea losing flowers after planting
First and foremost, this is the first plant I have planted…ever. I planted my Spirea as the expert instructed and for 2 or 3 days everything looked just fine. Then all of the white flowers were gone. I am left with just the nub of the flower. Is this normal after transplanting? Should I be doing something? Trimming the dead? Any help would be appreciated.

You will need to water a newly planted shrub daily for the first two weeks, after that you can cut back.
Your plant is also stressed so it may have dropped it's blossoms to save energy.
I would not trim anything on the plant right now, just work on getting it well watered and adjusted to it's new garden location.
Here is a link that list the care needed.
https://www.gardeningknowhow.com/ornamental/shrubs/spirea/growing-spirea-shrubs.htm