What is the size of the selection of content that you want to sync to the iPhone? The implication of the messages is that you want to sync an additional 42.45 GB over what is already stored on the device. Sometimes it seems iTunes can fail to correctly account for what is already there so gets this availably calculation wrong. Should that happen the easiest solution may be to remove what is currently on the device (Settings > General > iPhone Storage > Music > Edit > All Songs > Delete) and then add what you want from the library. Assumes your content is all in the library ready to put back.
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