Let's assume there is a polyline layer "line", see image below.

Try make_polygon($geometry)

Additionally try convex_hull($geometry)

Note: Mind the forms of your lines

P.S.: I can suggest inspecting the geometry menu in the 'Expression Dialog', see image below

Let's assume there is a polyline layer "line", see image below.

Proceed with RMT > Symbology > Single Symbol > Line > Geometry Generator

Use the following code make_polygon($geometry) in the "Geometry Generator"-field and change the Geometry Type into 'Polygon/MultiPolygon'.

Additionally try convex_hull($geometry) that will produce a Convex hull.

Note: Mind the forms/shape/guise of your lines.

P.S.: I can suggest inspecting the geometry menu in the 'Expression Dialog', see image below

Try make_polygon($geometry)

Additionally try convex_hull($geometry)

Note: Mind the forms of your lines

Let's assume there is a polyline layer "line", see image below.

Try make_polygon($geometry)

Additionally try convex_hull($geometry)

Note: Mind the forms of your lines

P.S.: I can suggest inspecting the geometry menu in the 'Expression Dialog', see image below