You are maybe forced to use a Solr version less than 7.3 with Java 10 or Java 11. If you try to start Solr you will receive this message:
Your current version of Java is too old to run this version of Solr
Here is how to fix it
If you do a fresh install of Solr 7 on Ubuntu 16 or 18 we will see several warnings after starting the service. This is a quick technical note on how to get rid of these errors.