radio tx bypasses the LoRaWAN mac layer and is preforming a raw TX of the data provided using LoRa modulation.
When the data is received it is not in LoRaWAN protocol format. Minimum size of a LoRaWAN packet is 12 bytes, hence the error “Frame too short”.
Use the “mac tx” command instead after performing OTA or ABP activation to communicate with the gateway.