yeah Melmetal (non-shiny) can be obtained pretty much at any date, but Melmetal (shiny) can only be obtained at certain dates.
So shinifying a non-shiny Melmetal (with the wrong dates, i.e. non-shiny encounter dates) will cause the legality error.
[If the date on the non-shiny Melmetal overlaps with a date that Meltan can be encountered shiny, it will not prompt the legality issue]
For starters, you're tagging people wrong. When you type "@" in the reply box, followed by the person's name, the "tagging" menu should appear.
For example:
Once the profile appears in the menu, you just gotta click it (to correctly tag the person. If you don't correctly tag the person, they won't get a notification.)
Well, I don't recommend creating mons from scratch.
Either grab a legal pk8 of a shiny Melmetal, or use the GP1->LGPE->SWSH method to get a legal pk8 generated by PKHeX.
Alternatively, if you want to shinify an existing Melmetal, just make sure the date obtained matches a date that Meltan could be caught shiny in GO.
[Also, my GO transfers upload, as well as my Shiny Living Dex upload, has legal shiny Melmetal]