Hello sfrederickson,
According to the provided screenshot you are trying to save a single TIFF file with about 500 000 x 700 000 pix dimensions. TIFF format is limited to 4 GB per file (which is usually about 30 000 x 30 000 pix), unless BigTIFF option is used.
Please make sure that the export resolution is selected correctly. If the project is scaled to real world dimensions, are you sure that you need sub-millimeter resolution? If you really need it, you may consider saving orthomosaic to several files using Split in Blocks option.