No apps linked with the "Open In" Functionality, therefore can't use it.

the odd thing is that in safari - I have no option = in chrome I do - so is there a browser setting somewhere? Also - how do I change from (say) text pad to text mate?

What type of file are you trying to open?

Does the file have an extension to it (i.e. a bit after the . such as .doc)

Thankyou … it’s a simple text file … .txt and .md

Dropbox doesnt have capability to open and edit text files I'm afraid. 

It is previewable only. 

https://help.dropbox.com/view-edit/file-types-that-preview

A couple of weeks ago when I clicked on the 'Open in' tab (see below) - it offered to open 'TextEdit' on my Mac - I could do that - make edits - and when it closed saved right into the dropbox. For some reason that functionality seems to now not be there. I know I didn’t imagine it.

That works if you are logged in to the computer that has Dropbox installed and running - as it then opens it directly in that app and program. 

Is Dropbox running on your computer now with no errors AND is the file available on your computer in the Dropbox folder. 

Thankyou for sticking with me ...

Yes - Drop Box - The App - is Installed and Running

I have opened Dropbox in the browser to the folder where an example file is stored

See image below

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FRACTALS wrote:

Yes - Drop Box - The App - is Installed and Running

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What is its status when you hover your cursor over the Dropbox icon in the menu bar?

It reads 

> Dropbox 181.4.5678

> Up to date

Hey FRACTALS! Hope you’re doing well. 

Were there any updates on the third-party app that you’re using to open this type of files, right before this issue started?

Is the file in question locally synced to your desktop app? Does it show a green checkmark, when you open your Finder > Dropbox folder?