TITLE: GCN CIRCULAR NUMBER: 29956 SUBJECT: GRB 210506A: Nanshan/NEXT optical upper limit DATE: 21/05/07 03:12:02 GMT FROM: Dong Xu at NAOC/CAS S.Y. Fu (NAOC), Z.P. Zhu (NAOC, HUST), X. Liu, D. Xu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report: We observed the field of GRB 210506A (Tohuvavohu, DeLaunay & Kennea, GCN 29947) using the NEXT-0.6m telescope located at Nanshan, Xinjiang, China. We obtained 9x300 s frames in the Sloan r-band, starting at 15:17:27 UT on 2021-05-06, i.e., 14.63 hr after the BAT trigger. No credible optical source is detected in our stacked image at the position of the XRT source #3 that lies in the BAT error circle (ref. https://www.swift.ac.uk/ToO_GRBs/00021430/ and Evans, GCN 29949), down to a limiting magnitude of r~21.7, calibrated with the nearby PanSTARRS field.