From that page you referred, there's actually a line stating an action is needed...
Run the following commands to install the Lego client. Note that you will need to replace the X.Y.Z placeholder with the actual version number of the downloaded archive:
In this case, you should run the following command:
Im presuming this is because I have already gone through this process before. Could you please advise next step from hear? Is it to go onto Step 3 in the above link or something else?
So it’s complaining about /opt/bitnami/apache2/conf/server.crt - does that file exist? What’s the output of:
ls -l /opt/bitnami/apache2/conf/server.crt
If it’s not there, if you or someone followed that tutorial before, there may be a /opt/bitnami/apache2/conf/server.crt.old that you can copy back temporarily just so you can go through the rest of the tutorial.
If you follow the rest of the tutorial, you'll see there's a step to remove it and replace it with a link to a valid certificate later. It just needs to be in place for that first part of step 2.