Paneldata Stata: what steps do I need to undertake?

I'm working on my master thesis and I need to use paneldata in Stata. I've done all the possible steps I think but is there any way of knowing that I missed some tests that are essential when you are working with paneldata? I also want to check the 'health' of my dataset, are there any test available for this? I also have some problems executing the Hausman Test to check if I need to use the random effects or the fixed effects model. When I execute this Stata shows me this message: "Note: the rank of the differenced variance matrix (7) does not equal the number of coefficients being tested (8); be sure this is what you expect, or there may be problems computing the test. Examine the output of your estimators for anything unexpected and possibly consider scaling your variables so that the coefficients are on a similar scale." I have no idea what this means. Can anyone help me with this?