MacOS Mojave Version 10.14.6 Newest version of Audacity
On my Macbook it is impossible to copy and paste text in filename and Metadata when saving a file as MP3
On my iMac running MacOS Mojave Version 10.14.5 and Audacity 1.3.14-beta (Unicode)
this is no problem and saves a lot of typing.
Thanks for letting us know about this. It is a known bug, but has not had much attention lately. I’ll bring it to the attention of the developers.
Have you tried right-click, then “Paste” from the context menu?
– Bill
I can’t reproduce this on my MacBook Pro with macOS 10.14.6 latest Mojave with either 2.3.2 or the latest alpha test build fir the upcoming 2.3.3
when trying to paste into the filename. So that bit works for me on Mac (and Windows)
I cannot (and never have been able to) paste into the metadata fields - nor can I ever on Windows - I think that’s a “feature” 
Peter.
I just updated to Catalina, and just installed 2.3.2 since my previous version is deprecated. I too can’t paste anything into the file name window when saving. Previously I had always been able to. I rely on this. My job is translating dialogue for TV shows and films, and recorded anywhere from ten to several hundred files a day. The file names are always unique (CHARACTERNAME_EPISODE/FILM_UNIQUEFILENUMBER.mp3), and I copy them from Final Draft document and paste them into Audacity so I don’t have to type it out by hand every single time. Being unable to paste into that window will add a significant amount of time to my work day—so much so that I might start searching around for other recording software. I don’t need high quality audio or bells and whistles: This is just for the actors’ reference. I beg the developers to please look into this again!
Please note that Catalina is not yet officially supported by Audacity.
See this Forum announcement Audacity under macOS 10.15 Catalina
And many thanks to noraa for posting that workaround
Peter
I am using Mac OS Catalina 10.15.4 and Audacity 2.3.2.
I confirm previous reports that on exporting audio the Copy and Paste functions do not work in the “Save As” field in the “Save” dialogue box when I use the keyboard shortcuts (Cmd C, Cmd V). However, I can work around this issue because these functions work fine when I right click on the mouse and select “Copy” or “Paste”.
This appears to be a problem only with the newer releases of Audacity as I never experienced this problem when using older versions of Audacity for many years.
Note that the keyboard shortcuts (Cmd C, Cmd V) work fine in the fields of the subsequent “Edit Metadata Tags” dialogue box, so the problem appears to be specific to the “Save” dialogue box.
As Billw58 pointed out earlier in this thread,
Thanks for letting us know about this. It is a known bug, but has not had much attention lately. I’ll bring it to the attention of the developers.
See: https://bugzilla.audacityteam.org/show_bug.cgi?id=1300
WC