Hi Fariha,
I reproduced your issue with following operation:
1. I created 2 A/B tests to marketing email: Subject test 1 and Subject test 2.
2. In customer journey, I selected Subject test 1, it passed error check.
3. I stopped the marketing email and deleted Subject test 1.
4. Back to customer journey again, Subject test 2 was automatically selected, but it will give same error as yours.
In a word, you may deleted the A/B test item that it had been used in customer journey.
Please delete the email tile and add a new again to test whether it could pass error check.
Regards,
Clofly