Hi Lisa,
I've looked into this further and it appears that the current publicly available version of Pro does not carry over line charts into ArcGIS Online and is currently a known limitation. However, other chart types such as Bar, Histrogram, etc. should work. ArcGIS Pro 2.9 will support carrying over line charts when it is publicly released.
Unfortunately, as of now, this is a known limitation. I will post any updates here once I hear that 2.9 is publicly available.
Thanks,
Ryan