You need to convert the track to one of the formats that is supported:
Wave (lossless) or Ogg-forbis (lossy) are the more popular, but when you click on the sampletrack, you will see all the formats, that is supported.
Inside LMMS large soundfiles are contained in the Sample-track
https://lmms.io/wiki/index.php?title=Wo ... s_as_loops
Obs, that sound-files can be HUGE, and you need a Big chunk of RAM!
Besides that, since you are new in Forum
Welcome joihnsonlee !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions