GRB 210308A: Nanshan/NEXT optical upper limits

We observed the field of GRB 210308A (D'Elia et al., GCN 29619) using the NEXT-0.6m optical telescope located at Nanshan, Xinjiang, China. Observations started at 14:16:29 UT on 2021-03-08, i.e., 7.67 hr after the BAT trigger. We obtained 15x200 s frames in the Sloan r- and i- bands, respectively. The optical afterglow (D'Elia et al., GCN 29619) of the burst is not detected in our stacked images, down to limiting magnitudes of r ~ 21.7 and i ~ 21.3, calibrated with nearby PS1 stars.