First confirm if you still have same status error:
If so, you can try change ownership of that file like @technicalpyro previously posted (with sudo this time):
sudo touch /var/lib/misc/dnsmasq.leases
sudo chown pihole:pihole /var/lib/misc/dnsmasq.leases
Restart:
sudo systemctl restart pihole-FTL
And check again:
sudo systemctl status pihole-FTL -l
Source of that dnsmasq.leases file is probably revealed if you run below one (dont skip the .*):
grep -v '#\|^$' -R /etc/dnsmasq.*