After you have updated your question I think I understand what you are doing but I think that you are not approaching the problem in the right way.
Firstly the way you define the system of equations introduces floating point coefficients and polynomial equations with floats can be extremely ill-conditioned so make sure to use e.g. S(1)/2 or Rational(1, 2) rather than 1/2 like this:
from sympy import *
x,t=symbols('x,t')
Coeffs = symbols('a11, a12, a13, a14, a15, a16, a17, a18, a19, a21, a22, a23, a24')
a11, a12, a13, a14, a15, a16, a17, a18, a19, a21, a22, a23, a24 = Coeffs
#E = expression in x,t,aij, 0<i<11,0<j<11 (nonlinear in aij)
## Sample
E = (
S(1)/2*(9*(8*t + 1)*a11
+ 3*(t**2 + 2*t + 1)*a21
+ 4*(t**3 + 3*t**2 + 3*t + 1)*a12
+ 5*(t** 4 + 4*t**3)*a13
+ 6*(t**5 + 5*t**4)*a14
+ 7*(t**6 + 6*t**5 + 1)*a15
+ 8*(t**7 + 7*t)*a16
+ 2*a17*(t + 1) + a18)*x**2
+ S(1)/48*(13860*(
252*(t**10 + 10*t)*a19
+ 1260*(t**2 + 2*t)*a21
+ 840*(t**3 + 3*t**2 + 3*t)*a22
+ 630*(t**4)*a23
+ 504*(t**5 + 10*t**2 + 5*t) )*a24
)
)
So you have E which looks like this:
In [2]: E
Out[2]:
⎛ ⎛ 10 ⎞ ⎛ 2 ⎞ ⎛ 3 2
a₂₄⋅⎝13860⋅a₁₉⋅⎝252⋅t + 2520⋅t⎠ + 13860⋅a₂₁⋅⎝1260⋅t + 2520⋅t⎠ + 13860⋅a₂₂⋅⎝840⋅t + 2520⋅t + 25
───────────────────────────────────────────────────────────────────────────────────────────────────
48
⎞ 4 5 2 ⎞ ⎛ ⎛ 3
20⋅t⎠ + 8731800⋅a₂₃⋅t + 6985440⋅t + 69854400⋅t + 34927200⋅t⎠ 2 ⎜a₁₁⋅(72⋅t + 9) a₁₂⋅⎝4⋅t +
─────────────────────────────────────────────────────────────── + x ⋅⎜────────────── + ────────────
⎝ 2
2 ⎞ ⎛ 4 3⎞ ⎛ 5 4⎞ ⎛ 6 5 ⎞ ⎛ 7
12⋅t + 12⋅t + 4⎠ a₁₃⋅⎝5⋅t + 20⋅t ⎠ a₁₄⋅⎝6⋅t + 30⋅t ⎠ a₁₅⋅⎝7⋅t + 42⋅t + 7⎠ a₁₆⋅⎝8⋅t +
───────────────── + ────────────────── + ────────────────── + ────────────────────── + ────────────
2 2 2 2 2
⎞ ⎛ 2 ⎞⎞
56⋅t⎠ a₁₈ a₂₁⋅⎝3⋅t + 6⋅t + 3⎠⎟
───── + a₁₇⋅(t + 1) + ─── + ────────────────────⎟
2 2 ⎠
The first term of E with a24 makes this nonlinear but only mildly so since it is quadratic and polynomial rather than say transcendental or something (you previously only described your equations as nonlinear which can mean anything).
You are substituting 100 different values for each of x and t and then trying to solve the 10000 equations to make E zero for each of those values. It is almost guaranteed that the only possible solutions for these equations are those that make all of the coefficents of the x,t polynomial zero. I guess what you are actually trying to do is find the values of the ai symbols that make E zero for all x,t but we don't need 10000 equations to do that. We can just extract the coefficients and solve those as equations:
In [3]: eqs = Poly(E, [x, t]).coeffs()
In [4]: eqs
Out[4]:
⎡ 7⋅a₁₅ 5⋅a₁₃ 3⋅a₂₁
⎢4⋅a₁₆, ─────, 3⋅a₁₄ + 21⋅a₁₅, ───── + 15⋅a₁₄, 2⋅a₁₂ + 10⋅a₁₃, 6⋅a₁₂ + ─────, 36⋅a₁₁ + 6⋅a₁₂ + 28⋅a
⎣ 2 2 2
9⋅a₁₁ 7⋅a₁₅ a₁₈ 3⋅a₂₁ 363825⋅a₂₃⋅
₁₆ + a₁₇ + 3⋅a₂₁, ───── + 2⋅a₁₂ + ───── + a₁₇ + ─── + ─────, 72765⋅a₁₉⋅a₂₄, 145530⋅a₂₄, ───────────
2 2 2 2 2
a₂₄
───, 242550⋅a₂₂⋅a₂₄, 363825⋅a₂₁⋅a₂₄ + 727650⋅a₂₂⋅a₂₄ + 1455300⋅a₂₄, 727650⋅a₁₉⋅a₂₄ + 727650⋅a₂₁⋅a₂₄
⎤
+ 727650⋅a₂₂⋅a₂₄ + 727650⋅a₂₄⎥
⎦
In [5]: %time [sol] = solve(eqs, Coeffs, dict=True)
CPU times: user 236 ms, sys: 8 ms, total: 244 ms
Wall time: 244 ms
In [6]: sol
Out[6]:
⎧ a₁₈ -4⋅a₁₈ ⎫
⎨a₁₁: ───, a₁₂: 0, a₁₃: 0, a₁₄: 0, a₁₅: 0, a₁₆: 0, a₁₇: ───────, a₂₁: 0, a₂₄: 0⎬
⎩ 63 7 ⎭
In [7]: checksol(eqs, sol)
Out[7]: True
This shows that the system of equations was underdetermined so e.g. a18 can be arbitrary provided a11 = a18/63 etc. If one of the unknowns is not listed in the solution dict then it can take any arbitrary value independently of the others.
I added a %time in there to show that it takes 244 milliseconds to solve this on my not very fast computer (on the current sympy master branch -- it took ~0.5 seconds with sympy 1.8). Note that I do have gmpy2 installed which can make various things faster in SymPy. You might get a speed up from installing sympy 1.9rc1 and also gmpy2 (pip install --pre --upgrade sympy and pip install gmpy2). It's also worth noting that some bugs for systems of this type were recently fixed in sympy so for other examples 1.9 might give more accurate results:
https://github.com/sympy/sympy/pull/21883