Query 2 tables without a join
In an almost literal translation from English to SQL, I would write the query as follows:
select guestNo
from guest
where guestName like 'Peter%'
and exists (
    select
    from booking
    where guestNo = guest.guestNo
    and dateTo is null
);

NB Postgres unlike some other DBMSes doesn't need to have any columns in the select clause! That's ideal in the subselect here as we're only interested in the existence of a record, not in its value).
See https://www.db-fiddle.com/f/tpzgVMwkQGAHBFxyMkyJvj/17