Sorta kinda. If you are going to do a workout in RGT it’s generally suggested you do it in Erg mode. That way your targets are set and it doesn’t matter what road you’re on.
If you do choose level mode for a workout, it’s best to do it on a flat course that way you can shoot for the targets using your gears as you would in a race.
Bottom line is it depends on what your goals are.
Hope this makes sense.
I am learning based on the other replies and believe when selecting just ride or magic roads without intervals in Erg mode gives you watts selection options since just ride doesn’t have a structured intervals planned, if you select workout with intervals Erg mode will follow the target power adjustments based on workout intervals settings. In other words when you ride using Erg mode without intervals you will be asked to setup target watts. Hopefully it’s clear and i am correct.
Ok, I had a play with all of this, this morning and what I get is not what you have described, unless we aren’t on the same page? I tried starting four different "Ride now " rides and within the options on the Sensor settings I had the usual ERG or level settings to choose from. My trainer, power,cadence and control were all being used by RGT using my Windows 11 laptop. ERG mode kept me at whatever watts I set regardless of cadence and level mode was the usual level mode where you needed to change gear/cadence to meet the targets. This was ok as you mentioned above. Now, I also set up an Endurance ride from the workout section of RGT(various power changes in the workout) and used ERG mode in the settings and that was exactly the same as above ie: it asked me for a target and kept me at that target?? going back to level mode I was back to setting the trainer intensity, which I left at 100% and to keep to the targets was a manual process using cadence and the gears. So in a nutshell, ERG in an RGT workout does not automatically follow the power targets in that workout? Whether this is different If I download a no-vid from Systm and run that I don’t know or ride a magic road etc.
Is it possible you don’t have Trainer Control toggled on ie green in the sensor settings?
This was my guess too, and yet it sounds like it is controlling it when he sets a target but please do check.
Thanks, it was on “green” when I set the settings so whether it was still on green as I started the ride I didn’t check. Also, ERG is asking for a wattage level, what would you set that at if you have trainer control enabled as surely then that is not needed? thanks for all of your help
Honestly, I HATE doing structured workouts on RGT or any sim app tbh (and Zwift, FulGaz, Rouvy etc) so much I avoid it almost at all costs. I will do some no-vid type workouts on a flat route cuz I’d do that outdoors too. But I also HATE no-vids cuz I have such a hard time getting my head into it.
Anyway, to attempt to answer your question, in my fairly limited experience with workouts and RGT I left the setting at 100W when using ERG. For me, that allowed the intervals to set the power required based on the intervals in the w/o. So, a MAP interval of 280 watts requires me to exert 280 watts, then the recovery interval of 115 watts releases that resistance to allow me to spin at 115. What I found was if I bumped up the ERG wattage setting, that resulted in an increase to the relative watts required in the different intervals. Does this make sense?
The setting imho is far from intuitive. If it was me, I’d use percentages and call it “ride intensity” like it’s done in the SYSTM app 
Yes, like m, I don’t have Zwift or anything else like that, more cost!! Plus cartoon figures pedalling along with 1980’s graphics are not my cup of tea. Also again like you, no vids do my head in so I think I am going to start sending them to RGT as that is better(just) than looking at the bland workout. That’s why I wanted to get ERG mode sorted in RGT. I will have another go next time I have a gap in my training and see if i can get ERG to work properly. Many thanks again