Here are all important links:
viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
There could be several reasons, but my guess is that you had a install problem, and now you have a flawed config-file
This is solved like: https://lmms.io/wiki/index.php?title=LM ... h_Settings
If it does not fix it, please post your:
* OS (+service-pack-version)
* lmms version