Module 'only assignments to symbols' error

GROUPS:
 I made this code in order to simulate a pdf using Metropolis-Hastings.  It works fine without "Module" but I need to use it as a function; when i do it does not work: Clear["Global*"]; Normalconmediayvarianzadesconocida[medida1_, medida2_, medida3_,    medida4_, medida5_, limiteinferiordemedia_, limitesuperiordemedia_,    limiteinferiordevarianza_, limitesuperiordevarianza_,    valorinicialparamedia_, valorinicialparavarianza_, pasoparamedia_,    pasoparavarianza_, iteraciones_] :=   Module[{Subscript[x, 1] = medida1, Subscript[x, 2] = medida2,     Subscript[x, 3] = medida3, Subscript[x, 4] = medida4,     Subscript[x, 5] = medida5, a = limiteinferiordemedia,    b = limitesuperiordemedia, c = limiteinferiordevarianza,    d = limitesuperiordevarianza,    Subscript[\[Xi], 0] = valorinicialparamedia,    Subscript[\[Upsilon], 0] = valorinicialparavarianza,    s = pasoparamedia, w = pasoparavarianza, m = iteraciones},  t1 = Table[Subscript[\[Xi], 0], {1}];  t2 = Table[Subscript[\[Upsilon], 0], {1}];  Do[U1 = RandomReal[];   \[Xi] = Last[t1];   If[U1 < 0.5, y1 = \[Xi] + s, y1 = \[Xi] - s];   U2 = RandomReal[];   \[Upsilon] = Last[t2];   If[U2 < 0.5, y2 = \[Upsilon] + w, y2 = \[Upsilon] - w];   r = \!$$\*UnderoverscriptBox[\(\[Product]$$, $$i = 1$$, $$n$$]\*FractionBox[SuperscriptBox[$$E$$, $$-\*FractionBox[SuperscriptBox[\((\*SubscriptBox[\(x$$, $$i$$] - y1)\), $$2$$], $$2 y2$$]\)], SqrtBox[$$y2$$]]\)/\!$$\*UnderoverscriptBox[\(\[Product]$$, $$i = 1$$, $$n$$]\*FractionBox[SuperscriptBox[$$E$$, $$-\*FractionBox[SuperscriptBox[\((\*SubscriptBox[\(x$$, $$i$$] - \[Xi])\), $$2$$], $$2 \\[Upsilon]$$]\)], SqrtBox[$$\[Upsilon]$$]]\) (1/(        b - a) (HeavisideTheta[y1 - a] - HeavisideTheta[y1 - b]) 1/(        Log[d/c] y2) (HeavisideTheta[y2 - c] -           HeavisideTheta[y2 - d]))/(1/(        b - a) (HeavisideTheta[\[Xi] - a] -           HeavisideTheta[\[Xi] - b]) 1/(        Log[d/c] \[Upsilon]) (HeavisideTheta[\[Upsilon] - c] -           HeavisideTheta[\[Upsilon] - d]));   U3 = RandomReal[]; If[U3 < r, y1 = y1, y1 = \[Xi]];    t1 = Append[t1, y1];   U4 = RandomReal[]; If[U4 < r, y2 = y2, y2 = \[Upsilon]];    t2 = Append[t2, y2], {m - 1}];  zz = Transpose[Append[{t1}, t2]]    Histogram[t1]    Histogram[t2]    Histogram3D[zz,      ColorFunction ->       Function[{height}, ColorData["Rainbow"][height]]]]Normalconmediayvarianzadesconocida[12, 13, 13, 13, 13, 0, 15, 0.2, 3, 12, 0.2, 0.2, 0.04, 28000]The original code, without "Module" (obs.: it works) is: Clear["Global*"]; Subscript[x, 1] = 12; Subscript[x, 2] = 13; Subscript[x, 3] = 13; Subscript[x, 4] = 13; Subscript[x, 5] = 13; n = 5; a = 0; b = 15;c = 0.2;d = 3;Subscript[\[Xi], 0] = 12;Subscript[\[Upsilon], 0] = 0.2;s = 0.2;w = 0.04;m = 28000;t1 = Table[Subscript[\[Xi], 0], {1}];t2 = Table[Subscript[\[Upsilon], 0], {1}];Do[U1 = RandomReal[];  \[Xi] = Last[t1];  If[U1 < 0.5, y1 = \[Xi] + s, y1 = \[Xi] - s];  U2 = RandomReal[];  \[Upsilon] = Last[t2];  If[U2 < 0.5, y2 = \[Upsilon] + w, y2 = \[Upsilon] - w];  r = \!$$\*UnderoverscriptBox[\(\[Product]$$, $$i = 1$$, $$n$$]\*FractionBox[SuperscriptBox[$$E$$, $$-\*FractionBox[SuperscriptBox[\((\*SubscriptBox[\(x$$, $$i$$] - y1)\), $$2$$], $$2 y2$$]\)], SqrtBox[$$y2$$]]\)/\!$$\*UnderoverscriptBox[\(\[Product]$$, $$i = 1$$, $$n$$]\*FractionBox[SuperscriptBox[$$E$$, $$-\*FractionBox[SuperscriptBox[\((\*SubscriptBox[\(x$$, $$i$$] - \[Xi])\), $$2$$], $$2 \\[Upsilon]$$]\)], SqrtBox[$$\[Upsilon]$$]]\) (1/(       b - a) (HeavisideTheta[y1 - a] - HeavisideTheta[y1 - b]) 1/(       Log[d/c] y2) (HeavisideTheta[y2 - c] -          HeavisideTheta[y2 - d]))/(1/(       b - a) (HeavisideTheta[\[Xi] - a] -          HeavisideTheta[\[Xi] - b]) 1/(       Log[d/c] \[Upsilon]) (HeavisideTheta[\[Upsilon] - c] -          HeavisideTheta[\[Upsilon] - d]));  U3 = RandomReal[]; If[U3 < r, y1 = y1, y1 = \[Xi]];   t1 = Append[t1, y1];  U4 = RandomReal[]; If[U4 < r, y2 = y2, y2 = \[Upsilon]];   t2 = Append[t2, y2], {m - 1}];zz = Transpose[Append[{t1}, t2]];Histogram[t1]Histogram[t2]Histogram3D[zz, ColorFunction -> Function[{height}, ColorData["Rainbow"][height]]]