What major technical hurdle must be overcome, requiring relay satellites, for successful crewed landings on the far side?

The lack of direct line-of-sight communication with Earth.

Conducting missions to the Moon's far side presents a significant communication challenge because the lunar body itself blocks any direct line of sight between the landing site and ground control stations on Earth. To maintain critical command, telemetry, and data links, engineers must deploy specialized relay satellites positioned strategically in orbits where they can maintain continuous contact with both the surface assets and Earth. Successfully overcoming this communication barrier is a vital component of establishing any long-term or crewed presence on the far hemisphere.