This error is caused when Akindi isn't set to open as an External Tool.
1. Go to "Content" and locate the Akindi link you created. Select the down arrow beside the Akindi link in order to open the pop-up menu. Then select "Edit Properties In-Place".
2. Ensure the option to "Open as External Resource" is checked. Once you've done this, can you try accessing Akindi again please and let me know if that works.
Here is a gif of full workflow: