Note: Since Android 6.0.1 is very old (released in 2015) and no longer supported by Google for security updates, you should proceed with caution. This article focuses on feasibility and legacy support.
If your Marshmallow device says "App not installed" during manual installation, your system partition might be corrupted. Use a computer: youtube apk android 6.0.1
The official YouTube app currently requires Android 8.0 (Oreo) or higher. For Android 6.0.1, the following versions were the last to offer functional compatibility before support was fully cut: Note: Since Android 6
How to Continue Using the YouTube App on Android 6.0 to 7.1 - wikiHow How to Continue Using the YouTube App on Android 6
From a security-research perspective, Android 6.0.1 should be considered a legacy-only platform unfit for general internet browsing, especially video streaming requiring persistent HTTP/HTTPS connections.
Bonus: Using a browser keeps you safer because you aren't installing outdated, unpatched APKs.