Hi,
This is how I would solve it:
In[270]:= Clear["Global`*"]
In[271]:=
data = {{27, 2.2}, {27.2, 2.2}, {27.4, 2.3}, {27.6, 2.3}, {27.8,
2.2}, {28, 2.2}, {28.2, 2.3}, {28.4, 2.2}, {28.6, 2.2}, {28.8,
2.3}, {29, 2.2}, {29.2, 2.2}, {29.4, 2.3}, {29.6, 2.2}, {29.8,
2.3}, {30, 2.3}, {30.2, 2.3}, {30.4, 2.3}, {30.6, 2.3}, {30.8,
2.3}, {31, 2.3}, {31.2, 2.3}, {31.4, 2.3}, {31.6, 2.3}, {31.8,
2.3}, {32, 2.3}, {32.2, 2.3}, {32.4, 2.3}, {32.6, 2.3}, {32.8,
2.4}, {33, 2.4}, {33.2, 2.4}, {33.4, 2.4}, {33.6, 2.5}, {33.8,
2.6}, {34, 2.7}, {34.2, 2.7}, {34.4, 2.8}, {34.6, 3.5}, {34.8,
5.1}, {35, 10.3}, {35.2, 15.4}, {35.4, 21.1}, {35.6, 29.1}, {35.8,
35.6}, {36, 38.4}, {36.2, 37.5}, {36.4, 32.8}, {36.6,
25.5}, {36.8, 17.5}, {37, 10.9}, {37.2, 6.4}, {37.4, 3.8}, {37.6,
2.8}, {37.8, 2.7}, {38, 2.9}, {38.2, 2.8}, {38.4, 2.7}, {38.6,
2.4}, {38.8, 2.3}, {39, 2.3}, {39.2, 2.4}, {39.4, 2.4}, {39.6,
2.4}, {39.8, 2.4}, {40, 2.3}};
In[272]:= \
(*pd=ListLogPlot[data,PlotStyle\[Rule]Red,PlotTheme\[Rule]"Detailed"]*)
In[273]:= nlm =
FindFit[data, pp (Sinc[a x - tt])^2 + q, {a, pp, tt, q, m}, x,
Method -> NMinimize]
Out[273]= {a -> -1.9653, pp -> 35.9368, tt -> -70.833, q -> 2.05044,
m -> -35.6927}
In[274]:= fitx[x_] = pp (Sinc[a x - tt])^2 + q /. %
Out[274]= 2.05044 + 35.9368 Sinc[70.833 - 1.9653 x]^2
In[275]:= (*fitp=LogPlot[fitx[x],{x,27,40},PlotStyle\[Rule]Green]*)
In[276]:= (*Show[pd,fitp]*)
I modified your equation took out the "m x" term and I used Sinc^2 instead Sin^2/x (not the same as Sin^2/x^2) not sure if you wanted that. "Method -> NMinimize" does the trick for this kind of fit. this is what I get: