This is self-compiled, and Roman's suggestion did the trick. Strange that Ubuntu 18.04 doesn't come with this installed.

Since we're here, can I ask what is the difference between Tcl and Tk? Up until today, I had assumed that they were the same package.

From: Pd-list <pd-list-bounces@lists.iem.at> on behalf of IOhannes m zmölnig <zmoelnig@iem.at>
Sent: 04 May 2018 09:52
To: pd-list@lists.iem.at
Subject: Re: [PD] PD won't run on Ubuntu 18.04

On 05/04/2018 06:15 AM, Liam Goodacre wrote:
> I can't get PD 0.48.1 to run on a fresh installation of 18.04 LTS. It compiled fine, but when I try to run PD I get:
>
> sh: 1: wish: not found

did you install the puredata package (from Ubuntu) or self-compiled?

because the "puredata" package depends on the "puredata-gui" package
which in turn depends on the "tk" package which should provide the
"wish" interpreter.

fasrd
IOhannes