What is the name of the user that you used when you tried to cd into the persistence folder ?
It has to be user openhab or a user that belongs to the group openhab. root will be able, too.
It looks like none of these criteria is fulfilled.
What you can do is to get a root shell:
sudo bash
then cd into that directory.
To delete the files I first would stop the openhab service. In case ZRAM is enabled also stop that one first.