Interplanetary transfers can only occur at specific times, when the planets are aligned in the optimal position. It can take months until the celestial bodies are in the right position for the interplanetary transfer burn.
If the space craft orbits Kerbin below 120 km altitude, Kerbal Space Program 2 slows down the warp speed to hundred times, so it can properly perform all orbital calculations, and displays the message:
Approaching Kerbin, lowering time warp to 100x for safety!
To enable higher continuous warp speed, raise the periapsis above 120 km.