MaGra Is it this app? https://play.google.com/store/apps/details?id=nz.co.rankers.freecampingnz
It opens fine for me on a 6a. There was a new version published on October 13. Have you tried reinstalling the app?
If it still doesn't open:
From where did you install it? Play Store, Aurora Store…
Is there a crash log, if yes, could you share it?
Are you trying to open it in a secondary profile / private space?
Are you using Play services?