Multiple problems
1. Rename Thanks to ThanksEmoteSound (just more descriptive)
2. I've never heard of player::playeraudio, but if it exists, I'm pretty sure the argument given should be the sound datablock name, Thanks (or ThanksEmoteSound if you follow #1)
3. You're condition check is wrong; your checking if they're admin, but it looks like you want to add a spam prevention instead. Change
%client.lastTime to
%client.lastThanksTime (again, just being more descriptive) and change the condition to
if($Sim::Time - %client.lastThanksTime > 10000) (change the 10000 to however many milliseconds you want)
4. The private add-on notification is completely unnecessary
serverPlay3d(soundDatablockName);
Yeah pretty sure you need to give it coordinates too