We have tried granting it with our admin user also but no avail.
This is how v16+ works in a community installation:
postgres=1 # create role cr createrole; CREATE ROLE
postgres 2=# set role cr; SET
postgres 2=> create role otherrole; CREATE ROLE postgres 2=> grant pg_signal_backend to otherrole; ERROR: permission denied to grant role "pg_signal_backend" DETAIL: Only roles with the ADMIN option on role "pg_signal_backend" may grant this role.
postgres 1=# grant pg_signal_backend to cr with admin option; GRANT ROLE
postgres 2=> grant pg_signal_backend to otherrole; GRANT ROLE
