This application uses the Jisho.org API to generate CSV and JSON files. These files can be imported to Kotoba from the Kotoba dashboard as custom decks.
The API rate limit has been set to 2 calls per second and one page call contains 20 results. So if your search has 3000 results, the process would take 75 seconds to finish. The fetching uses cors-anywhere so you will have to go to the page and enable it by clicking "Request temporary access to the demo server"
If you are getting a search error, that is most likely because too many requests have been sent to Jisho.org. You can't do anything about this, but wait it out.