Servicing Stack Update (SSU) for Windows 7

Solution 1:

I tried to manually install KB5004378 but everytime it failed with "The update is not suitable for your computer."

KB5004378 requires a Extended Security Updates (ESU) subscription. To put it simply, in order to install either update, you need a Windows 7 ESU Multiple Activation Key (MAK).

Source: Obtaining Extended Security Updates for eligible Windows devices

Installing MAK keys adds the ability to receive ESU. It doesn't replace the current product activation key (for example, OEM, KMS), nor does it reactivate the system. Organizations will have to install a new MAK key for every year that they deploy ESU.

Source: How can organizations install and activate the Windows 7 ESU MAK key?

The last SSU update that you can install without a ESU subscription is KB4536952