How to use repmgr with PostgreSQL 10?
The repmgr doc you linked to says to use wal_level = 'hot_standby' with PG versions 9.5 or older, and wal_level = 'replica' for PG versions 9.6 or newer.

# Ensure WAL files contain enough information to enable read-only queries
# on the standby.
#
#  PostgreSQL 9.5 and earlier: one of 'hot_standby' or 'logical'
#  PostgreSQL 9.6 and later: one of 'replica' or 'logical'
#    ('hot_standby' will still be accepted as an alias for 'replica')
#
# See: https://www.postgresql.org/docs/current/static/runtime-config-wal.html#GUC-WAL-LEVEL


You're using PostgreSQL 10, so replica or logical should be used for this option.