Sarah hasn't contacted me. She couldn't have got my message.
Suggests that the speaker believes or assumes that Sarah didn't receive the message, because if Sarah had received that message, she would have replied to the speaker.
The speaker is speculating as to why Sarah didn't reply. If the speaker wanted to express uncertainty then she/he would say:
- She might not have got my message.
- She may not have got my message.
- Perhaps/maybe she didn't get my message.
The modal verb could in its positive form expresses uncertainty
- She could have got my message
(there's a possibility Sarah received the message, the degree of uncertainty can be inferred by the speaker's intonation)
When could is used in its negative form it expresses a negative deduction or something that didn't happen in the past (couldn't + the perfect infinitive).
- She couldn't have got my message.
Here the speaker expresses the conviction that Sarah did not receive the message. The same meaning is conveyed with can't have + past participle as in
“she can't have got …”; “she can't have seen …”; “she can't have read …” etc.