According to this page, Laspy supports LAS/LAZ versions 1.0-1.4.
Looking at the Laspy code, it looks like
the format string "h6.0" would correspond to a version "1.6" of the spec.
A value of "6" for the format corresponds to a specific predefined header format. (See the code, or the following link, for clarification on what that means)
There's a good overview on the LAS format here (from a digital preservation standpoint) https://www.loc.gov/preservation/digital/formats/fdd/fdd000418.shtml. That link also covers what the 1-10 values mean for the headers
I don't see any mention of a version 1.6 on the ASPRS website. There it states that 1.4 is the latest official version. (I'm not sure of the current status of 2.x)
If someone knows of a spec for a version 1.6, then a fair bit of work would be needed in Laspy to support it.
Simply adding "h1.6" to the list of valid formats will stop that error, but will fail further down the line.. there are also version-specific tests when loading files and in the laspy tools themselves.
I think LasTools also supports a similar range of versions, which might explain why it can't read the version from the command line.
Given this is a binary file, it may be a bit tricky to 'fix' it if the tools can't read it. With text files like json or xml it's easy to forgive or ignore missing data; not so with binary.
If you created the file in the first place, try using either the "6" or "h4.0" formats instead.
If not, try going back to the source of the files to clarify what tools they used to produce it - it may be unreadable for the tools you have available.