GRB240421B: BOOTES-5/JGT optical upper limit

Following the detection of GRB 240421B by Swift (Williams et al. GCNC 36208), the BOOTES-5/JGT 0.6m robotic telescope at Observatorio Astronomico Nacional in San Pedro Martir (Mexico) automatically observed the GRB location starting on April. 21, 09:43:44 UT (~ 50 s after trigger). No new optical source is detected on the co-added images (20 x 1 s, clear-filter) within the enhanced Swift/XRT error box (Beardmore et al. GCNC 36213) down to 18.4 mag. which is consistent with both reports from UVOT (Kuin et al., GCNC 36214) and WINTER (Karambelkar et al. GCNC 36215). We thank the staff at Observatorio Astronomico Nacional in San Pedro Martir for their excellent support.