That’s the problem here. It is not in fact (for emphasis) installed in uefi. And you’re not booting it up in uefi.
To be sure (though I think you most likely are), first check if your windows is uefi.
Use windows (as admim) command “msinfo32”.
Verify it is in fact in uefi. We have to be very sure (even, as said, most likely it is).
When confirmed windows is in uefi, then boot up Manjaro in uefi  and do …okay, just use this link and just to be sure, at step 6, do the additional commands as well.
If you have more questions, it’s okay to ask. Be sure.
 - same link. Oh… link is in many places here.