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The message that the OpenSmalltalk VM emits when it cannot pthread_setsched(), has multiple solutions depending on the setup.
According to https://packages.ubuntu.com/groovy/util-linux the package util-linux for ubuntu has a command /usr/bin/prlimit
A method which you could try:
prlimit --rtprio=99 --pid=<pid of shell in which you will run squeak>
So you could try to raise/change the rtprio using prlimit.
Perhaps even the prlimit method is not working for you, but it is something that you could try to troubleshoot the issue.
If the prlimit method is not working , there may be some other software that disallows changing the rtprio.
David Stes