Is it different in safe/troubleshoot mode (hold Shift when launching TB)? There must be at least one recipient entered.

I apologise for wasting your time. I didn't know how to delete my question. I get an error msg "TLS version or certificate is incompatible. Will check with my organisation's tech support.

A TLS error might be due to the requirements imposed by TB 78:

https://support.mozilla.org/en-US/kb/thunderbird-78-faq#w_after-upgrading-to-thunderbird-78-i-cannot-receive-or-send-email-messages

fixed my server settings so no error msg on startup but SEND remains greyed out. No address books visible although the TO field is populated when I begin to enter a name

Safe/troubleshoot mode does not solve the issue.

Try running in Windows safe mode to bypass potential interference from startup apps like antivirus, and disable a VPN if you use one.