Data no showing correctly plot python

It's difficult to see where this fails, because we don't know what df["Frecuencia"] contains, and the mistake is probably in the creation of this. Consider providing a Minimal reproducible example next time.

What you want to achieve could be done like this, where frecuencia_relativa is a pandas Series.

import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame()

df["hijos"]=(0,5,6,0,2,4,0,4,10,15,0,6,2,3,0,4,5,4,8,7,3,5,3,2,6,3,3,0,2,2,6,1,2,3,2,2,4,4,4,6,1,2)
frecuencia_relativa = df.hijos.value_counts()/len(df["hijos"])

plt.bar(frecuencia_relativa.index, frecuencia_relativa.values)
plt.show()

EDIT:

It seems like the problem is that you created df["Frecuencia"] by

df["Frecuencia"] = df.hijos.value_counts()

This doesn't work, because it assigns the i-th values of the pandas Series df.hijos.value_counts() to the i-th row of df, resulting in this:

    hijos  frecuencia relativa
0       0             0.142857
1       5             0.047619
2       6             0.214286
3       0             0.142857
4       2             0.166667
5       4             0.071429
6       0             0.119048
7       4             0.023810
8      10             0.023810
9      15                  NaN
10      0             0.023810
11      6                  NaN
12      2                  NaN
13      3                  NaN
14      0                  NaN
15      4             0.023810
16      5                  NaN
17      4                  NaN
18      8                  NaN
19      7                  NaN
20      3                  NaN
21      5                  NaN
22      3                  NaN
23      2                  NaN
24      6                  NaN
25      3                  NaN
26      3                  NaN
27      0                  NaN
28      2                  NaN
29      2                  NaN
30      6                  NaN
31      1                  NaN
32      2                  NaN
33      3                  NaN
34      2                  NaN
35      2                  NaN
36      4                  NaN
37      4                  NaN
38      4                  NaN
39      6                  NaN
40      1                  NaN
41      2                  NaN

The values are thus in the wrong rows and therefore their positions on the x axis will be wrong, if plotted with plt.bar( df["hijos"],df["frecuencia relativa"]).