The guide to personalize Better Cities mentions a compatibility command to RTT, but typing StartQuest BBCRTTFix is not recognized.
Does anyone know why ?

First there is a BC thread to deal with BC Problems
Second, are you using the patch we made which holds the quest?
Third if you are using that patch, you shouldn't need to start it because it is enable by default. If you used the BBCRTTReset and wants to reverse it though then you can use the BBCRTTFix to re-enable compatibility.
Fourth the patch and the commands will only work on the newest version of Ruined Tails Tale. Not the old one. So you would need to update to v3+ to get the benefit of the patch.