Thank you to all for the quick responses. Now I know what I have to do. Regarding Radioddity Support at the same time as I posted this I sent them an email with the same question and here is the answer I received.
Thanks for reaching out to Radioddity Support! I am Leslie and I'd be happy to assist you with this.
Please kindly notice that the channels over 39 in DB20-G are receive-only channels, which can not be programmed as TX channels. That is why the radio has an error tone.
Here I attach the 500 Channels Table of DB20-G for your reference:
Channel 1-30 (RX and TX): 30 GMRS channels
Channel 31-39 (RX and TX): 9 DIY repeater channels
Channel 40-493 (RX only): 454 VHF(136-174MHz) and UHF(400-490MHz) channels
Channel 494-500 (RX only) 7 NOAA Channels
Please feel free to let me know if you need any further assistance or information.