I managed to solve my problem by adding the following line to my Service:
Environment="RASA_X_PASSWORD=test"
And the entire working service looks like this:
[Unit]
Description=Rasa Core
After=multi-user.target
Requires=rocketchat.service
[Service]
Environment="RASA_X_PASSWORD=test"
Type=simple
WorkingDirectory=/opt/rasaVicious/
ExecStart= /opt/anaconda3/envs/rasaEnv/bin/python3 -m rasa x -p 5006
User=root
Group=daemon
Hope this will help some people struggling with the same problem. Would be nice to be able to define the Rasa X password in a config file somewhere in the Rasa folder…