From https://www.naturalearthdata.com/about/terms-of-use/

All versions of Natural Earth raster + vector map data found on this
website are in the public domain. You may use the maps in any manner,
including modifying the content and design, electronic dissemination,
and offset printing. The primary authors, Tom Patterson and Nathaniel
Vaughn Kelso, and all other contributors renounce all financial claim to
the maps and invites you to use them for personal, educational, and
commercial purposes.

No permission is needed to use Natural Earth. Crediting the authors is
unnecessary.

iowever, if you wish to cite the map data, simply use one of the
following.

Short text:
Made with Natural Earth.

Long text:
Made with Natural Earth. Free vector and raster map data @
naturalearthdata.com.