Low power while maintaining high cadence
64 watts is well within the margin of error of 60w target.

Don't be afraid of going to your lowest gear on the trainer.

You should be working up to your target cadence, if you can't hold 90 without bouncing then you should aim for something lower. It will take time to get up there. 

60w is relatively low for most people, it may become easier for you to hold a higher cadence at more watts. 

Trainerroad philosophy is to focus on the power, especially in your intervals.

Bouncing and poor form won't help you get faster.