Booking.com upgraded their API Connection to fix a number of minor bugs.
This has resulted in errors being reported.
RATE_IS_A_SLAVE_RATE .
You have a 'child' rate in Booking.com that is calculated from another Rate. Previously Booking.com were not reporting the error.
Go to Booking.com Extranet, go to Rates & Availability and click 'Edit' on the Rate id identified in the error message, then 'Edit' the Price, and select 'Setup as a new rate plan'. Click Save.
View the details in this short video :- https://screenpal.com/watch/c0lV6CVmFNd