What are the consequences of not filing a 90-day report due to not having a TM30 in Thailand?

If you fail to do your 90-day report because you don't have a TM30, you will likely face a fine of up to 2,000 THB, which you can pay at immigration. Although you can still file late after the deadline, you will incur the fine for not having the TM30. While it may not pose problems for future entries into Thailand, any interaction with immigration will require you to have your TM30 documented. Options include staying in a hotel to obtain a TM30 or seeking proof of residence to address the issue.