Monitor mode enabled then gets disabled automatically

[brotai]

followed the instructions but get this error when I use sudo airmon-ng start wlan1:

PHY Interface Driver Chipset

phy0 wlan0 brcmfmac Broadcom 43430
phy3 wlan1 rtl88x2bu TP-Link Archer T3U [Realtek RTL8812BU]
(monitor mode enabled)

Error setting channel: command failed: Device or resource busy (-16)
Error -16 likely means your card was set back to station mode by something.
Removing non-monitor wlan1 interface...
(monitor mode disabled)

any ideas?
p.s: I have also tried sudo airmon-ng check kill, but to no avail
p.p.s:
sudo ip link set wlan1 down
sudo iw wlan1 set monitor none
also doesnt work...