In MissionPlanner / Config / Full Parameter list
on the right hand side, select Holybro S500 and Load Presaved
This will present an opportunity to check or uncheck certain params, but you will want to accept them all.
That suits a 4S LiPo.
You should already have battery voltage monitoring enabled and working before doing this.
Let us know if you build is different.
Otherwise download a .bin log file (dataflasah logs) and then you can upload it to a filesharing service like Dropbox or Onedrive, and paste the link in here for us to check it out.
No one can help much without a .bin log file.
If you are not getting to fly at all, then set LOG_DISARMED=1 to generate a log file.
Remember to set that back to 0 before long or you end up with giant logs you dont need.