Hi @SallyB and welcome.
Lots of advice above.
Another thought … when new to all of it (smart trainer, riding AND the app as well) I’d suggest just doing a few normal workouts, possibly not at full tilt to begin with, to get used to how all this stuff behaves together as a ‘system’.
Many of us here will have been through that early learning process before we had such good apps and so on and we’re lucky enough to get used to one thing at a time
One really basic advice thing: you don’t need to ‘chase’ numbers if you’re using erg. And it really is best not to.
All trainers are different as well, and then even trainers made my the same company but different models are different. Which means erg reacts faster or slower. But ultimately it works.
(There are some workouts that really test erg if I’m honest like Violator but let’s leave those ones alone for now - though you can make them work too)
Generally speaking while you’re getting used to it, just lower or increase cadence calmly and ignore what the screen says … the power will stabilise eventually. Remember as well - all through your pedal stroke you’re exerting different power and the trainer is aggregating that and averaging it but it does change from second to second. Let the trainer do it’s thing though and it’ll always catch up - and it doesn’t matter if you’re then fluctuating around that number fir the next minute or two in your interval - that’s normal.
Unless you switch (in the SUF app settings) power smoothing on, in fact you can never see the number consistently the same on some trainers are they’re reporting the vagaries of our pedal stroke so well. Smoothing (the app setting) actually can help here - all that does it average out what the app is getting from the trainer so you see an average power over (might be 3s) rather than instant. Doesn’t actually change what you’re doing just displays it differently.
In short, just pedal away, get used to how it al hangs together (so say, do some video workouts that maybe have less constant changes and do them at less than 100% so you can get used to the feel) and after a few it’ll start to become clear
Then you can get in to things like using your gears as well to ‘help’ the trainer and erg work faster. But I’d just play with it first and ask as many questions as you like …enjoy.
Ps. Thinking ahead to future questions - which trainer? (As they have their own foibles)