@SunflowerTed · Today 20:03
''Totally agree. The child shouldn’t dictate.''
I truly pity anyone who might have you as a step-parent...
''The child'' is actually a teenager who happens to be struggling with her parents divorce.
Back to you OP: It sounds like your partner's daughter is being pleasant and reasonable when interacting with you. She just does not want you to be a permanent fixture in her home when she is with her father for now.
He is not even legally divorced yet
If you are in your Late 50s you are not going to start a family together and it is probably wiser to keep separate homes and your independence.
If you are really in it for the long term his daughter will soon start getting more Independent and will be more interested in her friends and boyfriends anyway soon enough rather than clinging to her dad and things will become easier.
Of course you are going to get some criticism.
As mentioned he is not even officially divorced yet and his priority is rightly his children, not you.
Also you might also have missed another factor: he might not want to move things on to the next stage with you...and is therefore very happy to continue a relationship where you see each other regularly but do not leave together and where you are no regular involvement in his kids life, which is why he is not pushing his kids to accept you and he might be using her reluctance as a convenient excuse.
When someone divorces they often don't want to jump straight back into a committed relationship and living with someone else.