Hi Giovanni,
it would be very useful if you could post the actual code, not just a screenshot. Here is a great post on, well, how to post:
http://community.wolfram.com/groups/-/m/t/270507
It was quite difficult to read the screenshot, but I think that it all boils down to something like this:
Solve[6.34733 Sin[4.72654 x] - 0.546274 Sin[10.3090 x] == 0 && 0. < x < 10., x]
It appears that the equation has an infinite number of solutions,
so the additional constraint gives the roots in that interval. You still get the warning, but you do get results. When I typed exactly your equation in, I did not get a warning, but the calculation did not terminate within a couple of minutes. The warning just says that the coefficients are inexact numbers (Mathematica uses Machine Precision in your case), and that the algorithms behind Solve can only solve this with exact coefficients. Internally Mathematica converts everything and finds then solutions to the exact coefficients.
Cheers,
M.