Big Sur and Dropbox

I have a new 24" iMac running Big Sur. Dropbox was transferred from myl old iMac (Catalina) but will not open on the new computer. Dropbox does not seem to be available in the Apps Store. I have Reunion Touch on my iPhone so that I can access the files from Reunion 13 on my Mac, and Reunion Touch requires my files for Reunion be in Dropbox on my computer. Can anyone tell me what the problem is????