The problem is upstream from Plot. Look at DR for a typical value of T,
In:= DR[0.12, 1, 1, 0.08, 0.05]
The exponent has a number times a caret symbol.
I converted the inputs to simpler formatting (Cell menu - ConvertTo - RawInputForm) and saw an oddity.
B[Alpha_, Gamma_, Sigma_, t_, T_] := ((2*(Exp[Eta[Alpha, Sigma]*(T - t)] - 1))/
((Alpha + Eta[Alpha, Sigma])*(Exp[Eta[Alpha, Sigma]*(T - t)] - 1) +
Note "..Sigma]))*ˆ*(2*Alpha.." - the caret is treated as a separate symbol. Function A has a similar glitch.
The thing that you are using as a caret (^) is different than the shorthand for Power. I suggest retyping that character. The standard caret on most American keyboards is a Shift-6.
Since DR is not giving just a number, Plot cannot plot it.