Delete lines within polygon

You can use Select By location then delete selected rows in QGIS or join by within:

delete from ok_jl_riks_copy where id in (

select from
(select *
from ok_jl_riks_copy jl) sub1

inner join

(select wkb_geometry from ok_ak_riks ak) sub2

on st_within(sub1.wkb_geometry, sub2.wkb_geometry)

All lines within the polygons are deleted: enter image description here

Use an EXISTS expression:

FROM   <lines> AS ln
  FROM   <poly> AS pl
  WHERE  ST_Within(ln.geom, pl.geom)

If the ST_Within check hits the first TRUE (selecting a truthy 1), the sub-query terminates for the current row (no matter if there were more than one hit).

This is among the most efficient ways for when a table has to be traversed by row (as in an UPDATE/DELETE), or otherwise compared against a pre-selection (of e.g. ids).

As the error message says you should only return a single column, so try:

delete from lines_test2
 where id = ( select
 from lines_test2 a, straten b
 where st_intersects(a.geom, b.geom));

This assumes you have a unique ID column in your lines_test2 data table.