has this worked for you before?
Note that Dropbox only takes one image upon export, you will notice this for the Photos app too.
It will not show you any Dropbox icon in the share sheet by default.
The only way to accomplish this is by:
1. going to Files app
2. Tap on three dot button in the upper right, tap on Edit (will allow you to add Dropbox as a location)
3. Add Dropbox by enabling the toggle button next to it
4. Export multiple files from Lightroom mobile (Share as, Share to, Export to Files