Accomplished presenters don't ad lib and hope for the best, they read a script. Reading a script in an expressive and engaging way, without faltering and making it obvious that you're reading a script, takes a lot of practice. Practicing in front of family or friends does not adequately prepare you for presenting in front of an audience.
You can rehearse the same script
Rehearsing the same script over and over in front of an audience, helps you concentrate on rhythm and style
You don't have to use powerpoint slides
Presenting without slides requires a certain level of confidence and experience. The first time you do a presentation like that in front of us, you may find it a challenge, but the more you practice it, the more at ease you'll be with it.
You can even pretend you're standing at a conference lectern
Online practice sessions are very convenient and allow for plenty of practice. However, you needn't treat them like true online presentations. If you want to, you can stand up and practice a conference style presentation.
Making mistakes
If you mess up, get nervous or experience technical problems, then you've had a really worthwhile practice session because you will learn from that experience and next presentation you'll cope with the various situations better. We were going ot say 'or your mind goes blank' but you're reading from a script so that's not possible - see, scripts are good!