Why is the steps data in Health app all the same on everyday?
First thing to do is establish if the underlying data is wrong or the chart is wrong. 

Tap to the month (top of your picture) and the compare a few days with what Show all data reveals for the daily summaries. 


Strange steps measurement in iphone


The month view is shown in the thread linked above. 





Either way, I would back up your health data to iTunes with a passphrase to encrypt the backup and to iCloud so you have two good backups before you try to upgrade to the latest iOS. 

If the data is bad, you’ll have to look at the sources of step data and possibly upgrade the iOS / wait for a bug fix from Apple. 

If it’s just the graph, updating iOS and bug report to Apple is likely the fix.