Lion Air is fine, aircraft can be a bit shabby inside and out but it’s safe, cheap, and I’ve found them to be reliable and efficient.
Not uncommon for airline websites to block suspicious requests, I’ve had it before trying to book domestic flights from outside the country/region. If you use a VPN try setting it to Thailand, otherwise this has been the only reason I’ve ever used an OTA to book a ticket. Not a suspicious airline at all.