Im trying to assemble new matrices from some previous matrices I have created, but I can´t. What Im trying to do is the following:

1) I have different matrices:

K11e[1] = {{a, a}, {a, a}};
K11e[2] = {{b, b}, {b, b}};
K12e[1] = {{aa, aa, aa, aa}, {aa, aa, aa, aa}};
K12e[2] = {{ab, ab, ab, ab}, {ab, ab, ab, ab}};
K22e[1] = {{ca, ca, ca, ca}, {ca, ca, ca, ca}, {ca, ca, ca, ca}, {ca, ca, ca, ca}};
K22e[2] = {{cb, cb, cb, cb}, {cb, cb, cb, cb}, {cb, cb, cb, cb}, {cb, cb, cb, cb}};
K21e[1] = {{da, da}, {da, da}, {da, da}, {da, da}};
K21e[2] = {{ba, ba}, {ba, ba}, {ba, ba}, {ba, ba}};

2) Define 2 vectors, I will used them to assemble my global matrices:

L1 = {1, 2};
L2 = {3, 4, 5, 6};

3) With "Do" assemble n matrices (2 for this example) with the previous matrices

KE[z] = ConstantArray[0, {6, 6}];
KE[z][[L1, L1]] = KE[z][[L1, L1]] + K11e[z];
KE[z][[L1, L2]] = KE[z][[L1, L2]] + K12e[z];
KE[z][[L2, L1]] = KE[z][[L2, L1]] + K21e[z];
KE[z][[L2, L2]] = KE[z][[L2, L2]] + K22e[z];, {z, 1, 2}]

4) So Im expecting to get this matrices:

KE[1] = {{a, a, aa, aa, aa, aa}, {a, a, aa, aa, aa, aa}, {da, da, ca, 
  ca, ca, ca}, {da, da, ca, ca, ca, ca}, {da, da, ca, ca, ca, 
  ca}, {da, da, ca, ca, ca, ca}};
KE[2] = {{b, b, ab, ab, ab, ab}, {b, b, ab, ab, ab, ab}, {ba, ba, cb, 
  cb, cb, cb}, {ba, ba, cb, cb, cb, cb}, {ba, ba, cb, cb, cb, 
  cb}, {ba, ba, cb, cb, cb, cb}}

But instead Mathematica give me the next error:

KE[z] in the part assignment is not a symbol.

Many thanks in advance.

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You should use ArrayFlatten to build block matrices, for example

{{K11e[1], K12e[1]},
  {K21e[1], K22e[1]}} // ArrayFlatten

You can also define your own function if you want to apply it to multiple cases

myFunc = ArrayFlatten@MapThread[List, {{K11e[#], K21e[#]}, {K12e[#], K22e[#]}}, 1] &

myFunc@1 // MatrixForm



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