Removing the "Create a Site with Strikingly" footer is a Pro feature, and it's currently not available for Free and Limited accounts. Check out the Pro Guide!
Once you have upgraded to Pro, you can remove it by going to the site editor > Settings > Header & Footer > and unchecking the "Show Strikingly logo on the page?" box. Then, hit "Save".
Don't forget to "Publish" once you're done editing the site.
Let us know your feedback at support@strikingly.com or chat with us!