There are a couple of different reasons for this. And usually it happens in the evenings or weekends, when you are trying to make this conversion happen, so as to minimize disruption for your client.
- If you are trying to convert your QuickBooks Desktop file into a "Shell" that you've purchased, you have to do so within the first 60 days of the subscription being set up. If it is beyond the 60 days you can still do it, but you will first need to make a call to Intuit Support to "reopen" the file for you.
- You aren't the Master Admin on the QBO file.
So make sure that you check those two things before you try and complete the conversion on an evening or weekend, and save yourself some frustration.