On 28 Nov 2019, at 1:13, Nigel Chapman wrote:

OK, I fixed it by navigating to /Library/Application Support/TextMate/Ruby/1.8.7/bin in the Finder, clicking on the ruby executable, and then telling the system it was OK to open it, like you do with an unsigned app. A bit crummy, but it seems to have worked.

I expect disabling Gatekeeper would have worked too, but I don’t like to do that.

I don’t know why this happened – it may just be my setup and its history – but I’m posting this in case anyone else has the same problem.

I think it may have been because you moved the file from an old machine to a new one and somehow it got quarantined in the process and thus needed to be manually OK’ed.