I suggest NOT to use this library. No matter if your b4a version matches or not;
Dropbox has announced to stop the synclibrary in the next months...
Please find more infos about this
here. Additional you find the most up to date Dropbox-Library there.
But maybe you need to update your B4A.
A.) v2.71 is really old
B.) V5.8 is most up to date