PL/pgSQL anonymous code block

There must be an oid constant in ALTER LARGE OBJECT oid .... Try this workaround:

DO $$
    DECLARE
        bigobject integer;
    BEGIN
        SELECT lo_creat(-1) INTO bigobject;
        EXECUTE 'ALTER LARGE OBJECT ' || bigobject::text || ' OWNER TO postgres';
        ...

The same also applies to GRANT and REVOKE, of course.

In addition to what @klin already cleared up, you cannot use SELECT without a target in plpgsql code. Replace it with PERFORM in those calls.

Aside: Using "com.ektyn.eshops.myuser" as name for a role is a terrible idea. Use legal, lower case identifiers that don't have to be double-quoted.

This is an artifact of the fact that PostgreSQL has two completely different kinds of SQL statements internally - plannable (SELECT, INSERT, UPDATE, and DELETE) and unplannable (everything else) statements.

Only plannable statements support query parameters.

PL/pgSQL implements variable substitutions into statements, like your bigobject, using query parameters.

Because they aren't supported for non-plannable statements, no substitution is performed. So PL/pgSQL tries to execute the statement literally, as if you'd typed:

ALTER LARGE OBJECT bigobject OWNER TO postgres;

directly at the psql prompt. It does not detect this as an error.

To work around this, use EXECUTE ... FORMAT, e.g.

EXECUTE format('ALTER LARGE OBJECT %s OWNER TO postgres', bigobject);

See this related answer about COPY.