What does Lady Macbeth fear will be an obstacle to the commission of the crime and what does she decide to do about it?from act 1 scene 5 of Macbeth

If we look at Act I scene 5 carefully, we can see Lady
Macbeth's response to hearing about the prophecy of the witches and her concerns about
what might prevent Macbeth from gaining the crown that the witches have promised him.
The answer to your question can be found in the soliloquy that Lady Macbeth
gives:

Yet do
I fear thy nature:

It is too full o'th'milk of human
kindness,

To catch the nearest way. Thou wouldst be
great;

Art not without ambition, but
without

The illness should attend it: what thou wouldst
highly,

That wouldst thou holily; wouldst not play
false,

And yet wouldst wrongly
win...

Lady Macbeth then
obviously sees the potential for gaining power and prestige, but she is also aware of
her husbands "failing" in that he is too overcome by moral scruples to do anything
untoward that might help the prophecy come to life. So it is that Lady Macbeth wants her
husband to come back swiftly so that she can "pour my spirits into his ear" and chastise
and encourage him to do what she feels needs to be done to help the prophecy come
about.