The server issued a response status code that triggered automatic throttling in Burp Scanner. This can prevent the scan from running properly and is likely to impact coverage.
By default, Burp Scanner incrementally adds a short delay between requests until it complies with the server's rate limit. This enables the scan to continue as normal, but increases its overall duration. You can disable this behavior using a custom scan configuration. To do so, go to Request throttling configuration and deselect Automatic throttling.
You don't need to do anything, Burp Scanner attempts to resolve this issue automatically by default.
To prevent this from happening again, you can use a custom scan configuration to manually set a suitable delay between requests. To do this, go to Request throttling configuration > Throttle request interval.
If you're scanning one of your own sites in a test environment, where you aren't concerned about impacting the website's performance for other users, you may be able to disable server rate limiting and allow the scan to run at full speed.
Although Burp Scanner attempts to reduce the rate of requests it sends, if the server uses IP-based rate limiting, you may also need to throttle any other automated tools that are sending requests to the server from the same address