It looks like this is typical of newer riders like me, but checking:
On the FTP builder, just finished the first four rides, and really struggle to hit the cadence. Meanwhile, my power is crazy high. For instance, I’m supposed to be warming up at 85 rpm at 55 watts. For me to hit 85 rpm, I’m hitting 80+ watts. At 90 rpm, the bike starts shaking. Is this just a need for continued conditioning? I’m getting a bike fitting in two weeks. (I’m using a Saris M2. I upgraded the firmware and calibrated it before starting using it a few weeks ago.)
In my experience, hitting these cadences (80-95) are a bit strange at first, and you’ll feel like your pedaling [insert something that spins fast here]. But a cadence of ~90 is often considered the most efficient, balancing muscle use and lung/oxygen use.
However, it’s not the same for everyone. Personally, I’m between 80-100, going 90-100 during races for higher power output (cadence*torque=power). Warming up I’m usually a little less (70-85) at first, before getting loosened up and able to get into the range.
As for the actual workouts… I have felt the same thing with cadence requirements (one workout I dabbled in asked for 110rpm for a high wattage, and I rarely go above 100 
). It’s important to find a balance- a lot of the time, the workouts will be suggesting rpms that are actually a good idea, such as what was suggested for you during warmup/low-powers. Other times (IMO) the cadence it suggests is bonkers (for all I know, it’s a good idea, but I rebel).
But don’t take my word for it- do some research, online and with yourself on your trainer, to incorporate what is physiologically sound and what you find good to create the what you find best for you.
Another thing that might help is making sure you have ERG mode turned on during workouts. ERG changes the resistance to make sure you’re creating the right amount of power, no matter what cadence you do.