Аннотация:MASTER-Tavrida robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, SAI Crimea astronomical station) started inspect of the Fermi GRB210421.45 (trigger No 640695289,18h 08m 19.20s , +57d 09m 00.0s, R=2.13) errorbox 63 sec after notice time and 29500 sec after trigger time at 2021-04-21 19:06:25 UT, with upper limit up to 19.2 mag. The observations began at zenith distance = 59 deg. The sun altitude is -23.8 deg. The galactic latitude b = 28 deg., longitude l = 86 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1598312 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________| _____________________|__________ ___________|_____________________ _______________|_____|_______|___ ____|________ 29590 | 2021-04-21 19:06:25 | MASTER- Tavrida | (18h 05m 57.66s , +57d 18m 06.1s) | C | 180 | 18.5 | 29770 | 2021-04-21 19:06:25 | MASTER-Tavrida | (18h 05m 57.68s , +57d 18m 05.9s) | C | 540 | 19.2 | Coadd 29790 | 2021-04-21 19:09:44 | MASTER-Tavrida | (18h 06m 04.40s , +57d 18m 11.5s) | C | 180 | 18.5 | 29989 | 2021-04-21 19:13:04 | MASTER-Tavrida | (18h 06m 00.51s , +57d 19m 50.6s) | C | 180 | 18.7 | 30189 | 2021-04-21 19:16:23 | MASTER-Tavrida | (18h 06m 00.35s , +57d 18m 29.6s) | C | 180 | 18.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.