Nvm I figured out a different solution
A different solution? Do tell... Personally I had to work the equation backward but I figured out the steps that are needed include the photograph, her grinding on his lap, the diaper position on the desk, remember you can switch to each any position without it costing an AP point it's only when you do an action that it does, that said using the dildo on her in that position and popping her butt with a paddle over the desk are the other actions needed
1)Photo
2)over desk paddle
3)Diaper position Dildo
4)Grind while on chair
5)Dildo while over lap
6)Penetration while on chair
(These should be the steps you're looking for if I remember correctly)
I had this happen as well. Failed the first time on most recent play-through first time (a little buzzed) then replayed it using v15 beta walk-through and that also failed. Pretty sure the first failure was user error, but was surprised at the second. It can be done putting her in diaper position and taking no actions in that position though.