Many pages already out there telling you about this Mongo error. Have a simple search and look yourself.
Have a read of this stackoverflow page and it will give you the easiest answer.
https://stackoverflow.com/questions/52165333/deprecationwarning-buffer-is-deprecated-due-to-security-and-usability-issues
This is not an error, it is a warning that this method of creating a buffer is deprecated:
nodejs.org/api/buffer.html#buffer_new_buffer_string_encoding – stdob– Sep 4 '18 at 11:45
https://github.com/yarnpkg/yarn/issues/5770
https://nodejs.org/api/buffer.html#buffer_new_buffer_string_encoding
https://github.com/gatsbyjs/gatsby/issues/13094
https://nodejs.org/api/deprecations.html
https://nodejs.org/api/deprecations.html#deprecations_dep0005_buffer_constructor