The imagecreate function is part of php's gd library. If a server has been set up manually it's possible it doesn't have the gd library.
First, figure out which version of php you are using, for example with this command:
dpkg -l | grep '^ii' | grep php
Look for a line like:
ii php7.2 7.2.19-0ubuntu0.18.04.2 all server-side, HTML-embedded scripting language (metapackage)
That indicates the server is using php7.2. To install gd library for php7.2 on a system managed with apt you need a command something like:
sudo apt install php7.2-gd
And then the error will be gone.