The first mistake is building before you have a method. A platform cannot rescue a programme that is still changing every week. Run one cohort by hand, with a shared document and a call, then build the course around what worked. The lessons you put in Learnomy should be the ones you have already taught more than once.
The second is putting everything in the course. Coaching works because of the live part, and the live part is hard to put in a lesson. Use the course for material that does not change, the space for conversation, and sessions for what only a person can do. When clients ask for more live time, that is a good sign; add an office hours slot before you add more lessons.
The third is losing clients at the end. When a cohort finishes, many coaches let the space go quiet. Keep an alumni space open and run something regular in it, even a short monthly session. Clients who stay connected are the ones who come back for the next programme or recommend you to a friend. Finally, tell clients where each thing lives. A one-page start guide that says course here, conversation there and booking there saves a lot of messages.