Thanks for the replies.
Reaper Start Measure is set to start at 1

Setting the Sync Bar Offset to 29 now allows Reaper start to sync BIAB VST. So at least I can use this for now.

The problem is that every song will have to have a different offset.

In addition, it seems that when I engage Reapers Metronome, no matter what I set Count-in Measures to in Reaper, it no longer properly syncs at start. There is a lag of 1 to 3 measures before BIAB starts. I have tried this with 1 and 2 measures in Reaper, and adjusting the BIAB Sync Bar offset up and down by 1.

So the best I can do is not use a count-in measure but at least I can now jam using the BIAB VST.

I purchased BIAB a couple of years ago when the VST was first introduced and felt like I was beta testing it, so I haven't touched it until I purchased the 2023 version a couple of days ago. I am hoping that the current problem is fixable on my system.

Thanks again for the replies to get me going.