Faced same issue. Not exactly sure why it was not working. In my system there was just one executable named parallel in system path, but still it was not getting detected.
**But anyway, I was able to fix it as below**:
1. Run `whereis parallel`. This gives all the paths where executables named **parallel** is present. For my case there was just one path `/usr/local/bin/parallel`. Running using this path works just fine. 2. You can add an alias for this in `~/.bashrc` or `~/.zshrc` file like `alias parallel='/usr/local/bin/parallel'`
And now `parallel` works like charm.
dev-dsk % parallel --version GNU parallel 20190322 Copyright (C) 2007-2019 Ole Tange and Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. GNU parallel comes with no warranty.

For me it was same issue but different problem. Just running `parallel` command was exiting silently. Also `parallel --version` was saying `invalid option` error. In my Path there was just one parallel executable binary but still it was not detecting.
**I was able to fix it as below**:
1. Run `whereis parallel`. This gives all the paths where executables named **parallel** is present. For my case there was just one path `/usr/local/bin/parallel`. Running using this path works just fine. 2. You can add an alias for this in `~/.bashrc` or `~/.zshrc` file like `alias parallel='/usr/local/bin/parallel'`
And now `parallel` works like charm.
dev-dsk % parallel --version GNU parallel 20190322 Copyright (C) 2007-2019 Ole Tange and Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. GNU parallel comes with no warranty.