The message is telling you that iTunes has been asked to add more media that the phone has space for. It has nothing to do with the storage on your computer.
So if there is 38 GB available on the phone (note, available space as determined by iTunes, not you), this means that iTunes is trying to add 44.54 GB of media to it.
The first thing to do is see what iTunes says is available. Connect the phone to iTunes and look on the Device tab:
That will display a screen similar to the screenshot below. Look at the bottom of that panel, for the Capacity Bar. What are the various numbers and especially, what is the Other figure (the deep yellow section just to the left of the Free figure). If necessary, hover over the section on the bar to reveal the figure:
If the Other figure is larger than it should be, we can probably sort that out. But if it's simply that there isn't enough space on the phone, the solution will be to deselect some media (podcasts, videos or music) so that there is enough room on the phone.
If there isn't enough space on the phone to synchronise everything, then there is a way to deal with that, which I can explain if we get to that stage.
