while running the migration wizard, the users i select from the source AD are unmatched even though they exist in the target office 35 tenant. please what is the cause of this and what is the resolution?

In order for the migration wizard to match to Office 365 accounts there must be a common value for the application to match upon.
The wizard will look at the smtp address, mail alias, and user principal name values from the on-premises object and attempt to find a matching object in Office 365.
If the on-premises mailboxes have not been sync’d to Office 365 or if the email addresses or UPN are not sync’d up then there will not be a value that can match the on-premises objects to the cloud.
An alternative to this is to ‘pre link’ the objects.  This can be done by capturing the ObjectID property of each of the Office 365 accounts and adding this value to the on-premises user object in its proxyAddresses list as follows:

• O365OID:object-guid-of-O365-account-here

When the migration wizard is look for matches it will always first look to see if the on-premises account has a value like this and if it does do does not need to do a search and merely selects the Office365 object that has that guid value.
Note that there can still be cases where the matching is unable to occur.  There can be ambiguous objects in the cloud and in such a case the app cannot make a definitive 1-to-1 match and so it will leave it unmatched.
You can also browse for matches yourself using the browse button.  In the dialog that appears you will enter a few characters of the display name or user principal name of the target object you wish to find.  Found objects will appear in the resulting list and you can select the matching object.