Try installing the package via Windows Command Line using trusted-host.
"C:\Program Files\JMP\<JMPPRO|JMP>\<version#>\jmp_python.exe" -m pip install --trusted-host=pypi.org --trusted-host=files.pythonhosted.org --user <module name>
E.G., installing numpy within JMP Pro. "C:\Program Files\JMP\JMPPRO\18\jmp_python.exe" -m pip install --trusted-host=pypi.org --trusted-host=files.pythonhosted.org --user numpy
Caution: note that this is insecure and can lead to other issues. The error occurs when attempting to access a package repository behind corporate firewalls. Please evaluate with your IT department before considering this as a solution.