Keyboard and trackpad not responding after replacing trackpad flex cable

Make sure the cable is fully seated. It can be a bit tricky getting the cable seated fully at the Trackpad side. Make sure the latching lever is also fully engaged.

Was the cable brand new?

The cable can be easily damaged as well.

Or maybe it was not a cable failure, but an issue with the Trackpad itself, or possibly the keyboard or even Logic Board. The cable is normally the weakest link. Unfortunately you usually need to test with known good components to figure out which one is the source of the problem. Intermittent issues are always difficult to troubleshoot.

You can use an external USB keyboard to boot into the Apple Diagnostics.

I don't do repairs every day. For me, it is typical to have to do the repair procedure more than once, because things inside modern Macs are very tricky to get just right.

so my suggestion is to perform the repair process again, taking extra care that every step is perfect.