Hi @ManuelF.
just try with my steps with Ubuntu-18 as below-
Step01
Before going to download Elasticsearch file we need to install java
Because Elasticsearch uses java for internal process.
And also we need to set home path for java as below command.
Command :-
$sudo apt update && sudo apt install openjdk-8-jre-headless
$export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
java under installation process
set java home path-
Step02-for Elasticsearch
wget -qO - https://artifacts.elastic.co/GPG-KEY-elasticsearch | sudo apt-key add –
echo "deb https://artifacts.elastic.co/packages/5.x/apt stable main" | sudo tee -a /etc/apt/sources.list.d/elastic-5.x.list
install Elasticsearch now by using below command
Command :-
sudo apt update && sudo apt install elasticsearch you can select any version here
after complete above step-
we can start Elasticsearch as below command.
**Command :-**
sudo systemctl start elasticsearch.service
now we can verify the Elasticsearch running status by using beow command
**Command :-**
sudo systemctl status elasticsearch.service
test Elasticsearch as below -
sudo curl -XGET 'localhost:9200/'
Pls do like , if this is helpful to you 
Thanks
HadoopHelp