Is kernel 7.40 not comapatible with NW 7.40, please can you share which kernel versions are supported ?
No, that is not the problem. Kernel 7.40 is compatible with NW 7.40.
You can check PAM for Kernel for EHP4 ERP6:
https://apps.support.sap.com/sap/support/pam
If you check PAM EHP4 ERP6 is not compatible with Kernel 7.40.