Hey there, I think I need some more information to understand what the issue(s) is (are).
Assuming you are in ERG mode from your post. You say the workouts require you to pedal as speeds you can’t maintain.
Many of the workouts have a wide variety of cadence targets and depending on the workout, and depending on where in the workout you are, those targets can vary as well. Climbing focused workouts (like G.O.A.T. and Power Station) often have us at low cadences of 50-70 but also at high power at or above our FTP while other workouts might have us at cadences from 90-100+
So, with that, I am curious which workout you were doing where you couldn’t maintain the high cadence? Has this happened in other workouts too? Which ones? Do you know what your typical cadence is when you ride outside?
Also, as someone mentioned above, if you are using ERG mode, your trainer will create the power target for you and it will keep that target regardless of what cadence you choose. So, if the app gives you a target cadence of 90 and a target power of 165 watts (your FTP), it wouldn’t matter what your “actual” cadence is, the power target will stay fixed. Also as mentioned above, cycling at higher cadence is something that can be learned and practiced.
Staying in ERG will allow you to practice and increase your comfort at higher cadence over time.
Here is an interesting post on cadence worth a read from Sir @Coach.Mac.C