Being able to self pace for a 20min FTP test is a tough skill.
Even if indoors under constant load, knowing what number to hit on the power meter requires knowledge of your prior performance and where you expect to be now.
At least early on, you might benefit from a Ramp test if your equipment supports it. With this form of test the indoor bike sets an ever increasing power level, and you just keep turning pedals until you can't any longer.
So no pacing experience required.
To give you some idea mine looks like
this. (Note: mine won't work for you. It's for a lower power output/me !)
After several frustrating and failed 20min attempts I switched to the Ramp Test. This gave me usable and better yet, repeatable numbers.
All the best
Barry
I
f you want more details on my failures