It could well be that your file is too large for a BluRay disc.
25 gigabyte BluRay discs actually only hold 23.31 gigabytes of data. So it may be just a bit too tight a squeeze.
As a rule of thumb, you shouldn't try to squeeze more than 2 hours on a standard BluRay.
The file is to big for the disc, but that is why the fit content to available space tick box is there.
It does show that the size will be reduced and what the new reduced bit rate will be.
The problem is that the error message does not remove it's self so the burn button does not activate.
The program also has a limit to how far it will compress your file.
Have you tried cutting the project in half, as an experiment, to see if it will Fit to Disc when it's a smaller file?
There is a window of allowable Bit-Rates for BD, and PrE will never go above, or below that window. The determining factors of the resultant file size (what will be written to the disc), are the Duration, and the Bit-Rate, which must be within that BD-legal window.
As 2 hours seems to be a good Duration, I would take Steve's advice, and just Delete ~ 45 mins. of the Timeline, do a Save_As (never a Save), and test that Timeline with your gear. Using a rewritable BD-RE will save using up a blank BD for this test.