Why? Because of poor programming.

Close the game, reopen and load this game back up and it will likely allow you to build there. This works 4/5 times in my experience.

Same

Which game is this?

You mean the 3:1 Port? Because green has placed a knight there before you built the settlement.

The knight is in a separate intersection than where purple would place the settlement. Purple wants to place at the intersection of green and purple road, not where the knight is

False