Hi @m0jek,
The integration lives on the Splunk side, not the Nextcloud side — there is no Nextcloud app for Splunk. What you are looking for is a Splunk add-on that ingests Nextcloud logs.
This came up here before: Splunk App for Nextcloud — the links in that thread are dead now, but the add-on still exists on Splunkbase as an unsupported community app:
Since it is a Splunk-side add-on, questions about setup, compatibility, and missing features belong on Splunkbase or in Splunk’s community forums rather than here.
h.t.h.
ernolf