No handles with labels found to put in legend

It might be late but for anyone with the same issue the solution is using the method legend() for the corresponding ax not as for plt

fig = plt.figure()
ax = fig.add_subplot(111)
ax.spines['left'].set_position('zero')
ax.spines['right'].set_color('none')
ax.spines['bottom'].set_position('zero')
ax.spines['top'].set_color('none')
plt.axis([-5,5,-5,5])
ax.xaxis.set_ticks_position('bottom')
ax.yaxis.set_ticks_position('left')
plt.grid()
plt.arrow(0,0, 3,1, head_width=0.2, color='r', length_includes_head=True, label='u')
plt.arrow(0,0, 1,3, head_width=0.2, color='r', length_includes_head=True, label='v')
plt.arrow(0,0, 4,4, head_width=0.2, color='r', length_includes_head=True, label='u+v')
ax.legend()

You can explicitly define the elements in the legend.

For full control of which artists have a legend entry, it is possible to pass an iterable of legend artists followed by an iterable of legend labels respectively. Reference

Example:

arr1 = plt.arrow(0,0, 3,1, head_width=0.2, color='r', length_includes_head=True)
arr2 = plt.arrow(0,0, 1,3, head_width=0.2, color='g', length_includes_head=True)
arr3 = plt.arrow(0,0, 4,4, head_width=0.2, color='b', length_includes_head=True)

plt.xlim(0,5)
plt.ylim(0,5)

plt.legend([arr1, arr2, arr3], ['u','v','u+v'])

The error is thrown because you haven't specified the label text

Either do something like this

plt.hist([x01, x02,x03], color=["lightcoral","lightskyblue","slategrey"], stacked=True, 
             label=['Supressed','Active','Resolved'])
plt.legend()

Or

Do not use plt.legend() if you haven't specified the label text as in the following WRONG example:

plt.hist([x01])
plt.legend()

The above will throw the same error, so either remove legend function or provide what it needs -> label. Side note: Here x01 is just a list of number for which I am creating a histogram, in the first example they are three list of numbers to create stacked bar chart

The bottom line is this error is thrown because of not specifying legend text and calling/initializing a legend