Forum OpenACS Q&A: Re: Invalid command name "::thread::create", invoked from within ns_mutex eval ns_mutex::throttle
apparently, your installation lacks Tcl thread support https://www.tcl.tk/man/tcl/ThreadCmd/thread.htm
When one installs Naviserver from installation script here https://raw.githubusercontent.com/gustafn/install-ns/master/install-ns.sh, thread support should be compiled and enabled in your tcl installation via the --enable-threads configuration flag.
Just a wild guess: can it be that a different tcl version was installed e.g. via the package manager that is lacking thread support and this is overriding the compiled one? I would also check the log file at startup for possible hints.