The calibration error means we can't find or can't get access to the folder where the battery info is stored on your device.
If you are rooted:
Android versions 7.0 and later include SELinux security policies that inhibit access to files under /sys/class/power_supply/ that AccuBattery uses. Root access should allow you to add or modify SELinux policies to allow the AccuBattery app UID to access files under this sysfs subtree. This is, however, a complex matter that we cannot provide instructions or support for.