I think you meant LR CC 2015?
I just tried this and it works fine LR CC 2015 -> PS 2018 -> LR CC2015. It works ok with File/save and ctl/cmd S. I am on a Mac Book Pro/Sierra machine.
Did not mean 2015
I have updated to PS CC 2018 and LR CC 2018 (Classic). I have both iMac & MacBook Pro (both High Sierra)
Well, there is only LR CC 2015 and LR Classic (V7). No such app as LR 2018.
In any case both versions of LR work fine with sending an image to PS 2018 and back.
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when exporting picture from LR CC (2017) to PS 2018
Without knowing 'How' you are 'exporting'- This may be your problem.
Do not use the Export function with Post Processing in Photoshop.
You should instead use the "Edit In" method to open the image in Photoshop.
Menu > Photo > Edit In > Edit in Adobe Photoshop CC 2018
or [ Ctrl+E ]
If you 'Export' the image with PP "After Export" in Photoshop, Photoshop accepts the totally new image file that as yet has no destination. It only exists as a document in Photoshop. Hence "Save" is not available as Photoshop does not know where the file is to be stored. Photoshop is insisting on a location by forcing the "Save As" option.
Edit: There is the option in the Lightroom Export dialog to "Add to this Catalog" which would give you the "Save" option on Photoshop.