It is in the nature of a negotiation that a solution needs to be agreed by both sides. It may get amended a bit in the process, but the end result is shared by both.
We have NOT found such a solution. And Teresa May was saying we have a responsibility to try to find one both sides can agree on.

The point which needs to be made, yet again, is that negotiations have not yet been concluded.

All this discussion about "Post Brexit life in the UK" remains speculation at present. And anything which appears to have already been agreed is still open for negotiation if agreement can't be reached on other matters. It is the EU's insistence (written into their rules for dealing with UK withdrawal) that "Nothing can be agreed until everything is agreed"