"invalid NDK version -unable to find NDK version,please pick a different folder

hey i just downloadet unity in the newest version. In the installation i picked everything to install. When i create a new Project the message in the title is appearing. with the message there comes a download button.when i press download it installes a android ndk rar folder. i dont know what to do with the downloaded folder. i have windows 10 and the newest components. im using a 64-Bit system.
thanks for your help

Try watching this : Unity Android Tutorial : SDK, Build and Publish - YouTube . I use it whenever I install SDK Unity.