Аннотация:MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 210421C ( Fermi GBM team, GCN 29871) errorbox 3606 sec after notice time and 3636 sec after trigger time at 2021-04-21 21:13:29 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 24 deg. The sun altitude is -62.3 deg. The galactic latitude b = 1 deg., longitude l = 303 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1598370 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________| _____________________|__________ ___________|_____________________ _______________|_____|_______|___ ____|________ 3666 | 2021-04-21 21:13:29 | MASTER- SAAO | (12h 44m 20.32s , -55d 59m 18.1s) | C | 60 | 18.5 | 3839 | 2021-04-21 21:16:21 | MASTER-SAAO | (13h 00m 06.56s , -55d 58m 20.2s) | C | 60 | 17.6 | 3918 | 2021-04-21 21:17:41 | MASTER- SAAO | (12h 35m 41.98s , -53d 57m 24.5s) | C | 60 | 18.4 | 4248 | 2021-04-21 21:23:10 | MASTER-SAAO | (12h 47m 33.26s , -53d 57m 52.4s) | C | 60 | 18.8 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.