fsck doesn't run on ZFS. You shouldn't need to but you can scrub the zpool. See:
https://docs.netgate.com/pfsense/en/latest/troubleshooting/filesystem-check.html
For example:
[24.03-RC][root@1100-3.stevew.lan]/root: zpool status
pool: pfSense
state: ONLINE
status: Some supported and requested features are not enabled on the pool.
The pool can still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
the pool may no longer be accessible by software that does not support
the features. See zpool-features(7) for details.
config:
NAME STATE READ WRITE CKSUM
pfSense ONLINE 0 0 0
mmcsd0s3a ONLINE 0 0 0
errors: No known data errors
[24.03-RC][root@1100-3.stevew.lan]/root: zpool scrub pfSense
[24.03-RC][root@1100-3.stevew.lan]/root: zpool status
pool: pfSense
state: ONLINE
status: Some supported and requested features are not enabled on the pool.
The pool can still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
the pool may no longer be accessible by software that does not support
the features. See zpool-features(7) for details.
scan: scrub in progress since Fri Apr 19 14:06:16 2024
1.49G / 1.49G scanned, 0B / 1.49G issued
0B repaired, 0.00% done, no estimated completion time
config:
NAME STATE READ WRITE CKSUM
pfSense ONLINE 0 0 0
mmcsd0s3a ONLINE 0 0 0
errors: No known data errors
It can take a while but runs in the background.