Hello Noucky,
thanks for choosing Ikebana!
I’ve tested the theme with the latest version of WP and the Theme Option uploads do seem to work fine – during uploading, please make sure that the “Link URL” field in the upload popup window contains the path to the image file; in case it doesn’t, click the “File URL” button below the field.
Regarding the notice during plugin installation, I’ll need to know a couple of things in order to troubleshoot efficiently: does it appear on the back-end (admin) or the front end? Does it stay permanently there or disappear after page refresh? Does the plugin itself install successfully despite the notice or not?
Sincerely,
Andrii / Satori Studio