Why does Venus have no internally generated magnetic field?
In plain words
Venus has an iron core roughly as large as Earth's but no magnetic field of its own. Either its core does not churn today or something else stops a dynamo (a self-sustaining magnetic generator made by flowing liquid metal).
Precise statement
Spacecraft limits put any intrinsic Venus dipole moment below roughly $1e-5$ of Earth's (approximate, from Pioneer Venus era data). Determine why the Venus core sustains no dynamo: low core-mantle heat flow under a stagnant lid with no inner-core growth, a fully liquid or fully solid core, or a compositionally stratified core formed without a late giant impact (Jacobson and coauthors, EPSL 2017, https://doi.org/10.1016/j.epsl.2017.06.023). An answer is a core state (radius, solid fraction, stratification) and core heat flow consistent with the measured tidal Love number $k_2$, the moment of inertia and a thermal history, including whether a dynamo operated in the past.
What would settle it
Orbiter tracking that measures the Venus $k_{2}$ and moment of inertia precisely, combined with a search for remanent magnetization in old crust.