I am also able to run su - myuser in a script, but I receive the same error when calling it in a StackScript. It appears that this is related to job control not being enabled.

In an environment where job control is enabled, you technically can use a statement like that in a bash script - at least in that it may not generate an error - but it should not have the intended result, since subsequent commands would be returned to the user that first called the su command. In other words, the script would not know to run the commands as the user you switched to, but rather it would execute the su line and return execution back to the user that called the command on completion. There are 3 ways that I know of to use su in a script successfully.

#!/bin/bash

# switch user and execute a single command at a time with 'su -c'
su -c myuser 'command'
su -c myuser 'next command'
su -c myuser 'etc...'

# put the commands in a second script, and then invoke that
# script with 'su -c'
su -c myuser 'bash /path/to/script.sh'

# switch user and execute multiple commands, using a here document
# this will run commands from the beginning to the end of the
# document (from '<<CMD' to 'CMD'), and can also be used to automate
# input into commands, as below
su -c myuser <<CMD
command 1
command 2
input for command 2
etc...
CMD

# a more specific example, again using a here document and entering
# input for the user. This code actually will fail, as you cannot
# change a password this way in a here document, but it illustrates
# what I'm saying a bit more clearly
su -c myuser <<CMD
passwd
<new password>
CMD

As a quick side note, if you do wish to change a password from within a here document, you can do it like this

# generate a random password and store it in '$new_password', then
# change the password for 'myuser' to the newly generated password
su -c myuser >>CMD
new_password=$(date +%s|sha256sum|base64|head -c 32)
echo "myuser:${new_password}" | chpasswd
CMD

Thank you for your helo Tommy!