Hello, Thank you for reporting and providing the steps!
I reproduced the error on the demo site: https://smart1.openpanel.site:2087/user/edit/stefan
{"message":"Error executing command opencli user-change_plan stefan 4 : Error: Unable to fetch limits for the new plan ('').\n"}
Seems we introduced the bug in version 0.1.7 when we replaced plan_id with plan_name in the command because WHMCS does not support passing plan IDs. And I missed to change it here on the front-end.. 🤦♂️
Its fixed now and will be added in the next update in few days. In meantime please use the terminal command instead:
opencli user-change_plan tezilaw PLAN_NAME_HERE