I just installed Libre office and when I click on Base, I get a message saying "JrE required. Why
Because LO Base requires Java to operate (under most circumstances). In future please search for answers on a subject before asking. This question comes up multiple times per week.
What you need to install depends upon your OS & the LO you have installed. Please see my answer here for further info - click here.
If this answers your question please tick the (upper left area of answer). It helps others to know there was an accepted answer.