When mapping a 1..0 or 1 relationship in EF, the dependent relationship is expected to have its primary key be the foreign key of the principal.
In your case, this means that your Access
entity should drop the AccessId
primary key and make the UserId
be your key.
If you can't change your schema to allow this, your best bet for a workaround will have to be making the relationship a 1..many and fudging it in your code.