does reacting with tactical retreat prevent cull the weak ?

Yes — Tactical Retreat saves the unit. It's a replacement effect ("the next time it would die this turn, heal it, exhaust it, and recall it instead"), so it intercedes when Cull's kill happens and the unit is recalled to base instead of dying (Rules 369–370).

The practical outcome depends on how many units you have: with only one unit, the Cull resolves and its "each player kills one of their units" instruction can't be followed for you, so nothing dies (FAQ 7051 — you just ignore the instruction). With two or more units, you choose another one to die instead; Tactical Retreat only protects the one unit it was played on.

Note the timing window: you must play it as a Reaction while Cull the Weak is still on the chain, before both players pass priority and it resolves (FAQ 1432). This works because Tactical Retreat protects against any death, not just damage — unlike Unyielding Spirit, which Cull ignores since Cull isn't damage.