There is no Duolingo for Thai - but here are the alternatives
Unfortunately there is no Duolingo for Thai and it doesn't look like that's going to change any time soon. The two apps that I know that come kind of close are Drops and Ling:
They're not exactly great but Thai is not a popular language to learn so you'll just need to take whatever resources are out there. I slightly prefer Drops but you should give both of these a try and see which works better for you.
If you're learning Thai alphabet, I recently a started an AI account that writes an article about a different Thai letter every day. I set the bot up to write about each letter the way that makes it easy for me to learn it. First, it introduces the letter and its pronunciation, then provides some examples of common words with that letter. Finally, writes a simple sentence in Thai using each of the words and provides an English translation. I'm genuinely surprised how well it's worked for me so far. I'm finally starting to pick up some letters and I don't feel like it costs me a lot of effort.
Check it out here: https://www.sanity.media/u/learn-thai-alphabet