Quote Would be so grateful for some help before trying a reinstall.

I can't imagine that anything short of a full reinstall would have any effect unless you have a system backup made before the problem started happening.

Uninstalling PD won't delete any of your custom or downloaded items, just make sure you have the installer ready to go. If you don't have that, you'll need to contact Cyberlink customer service to have them send you a new one. See FAQ #1 here.

The red No go circle can appear if you drag the transition to the center or the audio portion of the clip. When that happens move the mouse upward ¼ inch or more to the video portion of the clips until the No go circle disappears. You can then release the mouse button to place the transition.

OP stated that the no-go circle appears as soon as the transition is clicked on in the Transition room. I have no idea what's going on but something appears to have gone wrong with PD's installation.

Answering my own query!
No idea what has happened but yesterday evening, after emptying temporary files and rebooting a couple of times, it started working again.
I still get the No-go sign when I click on it but it disappears now when hovering at the beginning or end of a clip and inserts the transition. It is quite a long clip with lots of editing and additions. Maybe my computer had difficulty dealing with it. It's not a top-end machine although plenty of space on the hard disk.
Thanks to everyone for their input.
All I need to do now is find out how to close the topic! ;

There's no "close" option and discussions will stay on the forum as-is for years.

If you feel that one or more people answered your question, as the OP you can click the appropriate option below the appropriate post(s):

Thanks for your help.