You should make sure both computers are in the same Workgroup/Homegroup and that you are logged in with the same username and password on each*. Do not leave the password blank. If the password is blank (not recommended) you must enable anonymous or Guest
access (again, not recommended), and the shared folder must have the "Everyone" group in both share permissions and access control list (don't do it).
Also, you may refer to Microsoft KB and check if it helps:
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