There seems to be a large number of questions surrounding the fitting of functions to data sets. Many of these have the same problems with invalid starting values, missing x-values, insufficient amount of data points, missing restraints. Many are closed off-topic, others receive simple answers and are never closed.
A preliminary Q&A is now posted here:
What are some common issues with fitting functions to data? If additional info is required feel free to add it to the answer, or post a new answer if it concerns different MMA functions.