You have a backup which is scheduled to start 'Upon time since last backup". You change time interval in this option.
Then you notice that Next backup status turns to Not scheduled.
Next backup time is calculated according to the new setting once fist backup after this changes completes.
You need to start first backup manually by clicking Run now after creating/updating backup plan schedule.
Acronis development team is working on improving product behavior in this scenario.
Once you have saved updated backup plan, click Run now. Start time of the next backup will be calculated and displayed upon first backup completion.