Too bad.. Running Linux Mint 18.3, based on Ubuntu 16.04 gives me:

$ ./start.sh
Running Undead Horde
./UndeadHorde: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.27' not found (required by ./UndeadHorde)

$ ldd --version
ldd (Ubuntu GLIBC 2.23-0ubuntu11.2) 2.23
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.
$ lsof -p $$ | grep libc
zsh 11271 xxuser mem REG 253,0 1868984 22544864 /lib/x86_64-linux-gnu/libc-2.23.so
zsh 11271 xxuser mem REG 253,0 23128 22548860 /lib/x86_64-linux-gnu/libcap.so.2.24
$ uname -a
Linux yypcname 4.15.0-88-generic #88~16.04.1-Ubuntu SMP Wed Feb 12 04:19:15 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Linux Mint 18.3 is no longer supported. You should either update your OS to newer version, or install needed software yoursef.