لَأُعَذِّبَنَّهُ عَذَابًا شَدِيدًا أَوْ لَأَذْبَحَنَّهُ

I will punish him with a severe punishment - 27:21

It is part of the political sagacity to punish the absentee after making due scrutiny.

It is permissible to punish a lethargic animal moderately

Allah Ta’ ala had permitted Sayyidna Sulaiman (علیہ السلام) to punish the animals in the same way, as He had permitted people of all ages, a permission which is still valid, to slaughter them to make use of their flesh, bones, skin etc. Similarly, if the domestic animals, such as cow, horse, camel, donkey etc., do not perform their normal duty, then it is permissible even now to punish them moderately for disciplining. Punishing animals, other than domestic, is not permissible under Islamic law. (Qurtubi)

أَوْ لَيَأْتِيَنِّي بِسُلْطَانٍ مُّبِينٍ

Unless he brings to me a clear plea. - 27:21

That is, if the hoopoe offers a plausible excuse for its absence, then it will be saved from the punishment. There is a subtle hint in it that it is expected of a ruler or an administrator that if someone falters doing something, then it should be thoroughly investigated before punishing him. The punishment should be enforced only when the guilt is established, otherwise he should be forgiven.