Circle of Spores druids gain the Fungal Infestation feature at 6th level. Its description states, in part (GGtR, p. 27):
If a beast or humanoid that is Small or Medium dies within 10 feet of you, you can use your reaction to animate it, causing it to stand up immediately with 1 hit point. The creature uses the zombie stat block in the Monster Manual. It remains animate for 1 hour, after which time it collapses and dies.
If a beast or humanoid (e.g. a bandit) dies within 10 feet of a Circle of Spores druid, and the druid doesn’t use Fungal Infestation right away to animate them as a zombie (e.g. if the druid already spent their reaction earlier in the round, or whatever), can the druid wait until their next turn to use a reaction to animate the dead creature as a zombie?