Hi Jagdish,
Further to Giovanni's suggestions, there was a recent Dropbox issue, MDL-70268, that was fixed in Moodle 3.9.5 and 3.10.2. What version of Moodle are you using?
General help
Dropbox enabling
Dyskusyjŏ ôstała zaszperowanŏ, tōż niy możesz już ôdpedzieć.