Dear @jaakkor2, thank you for the answer - its works !
Just for the record, for others reading this post, the “Manifest.toml” to edit is that one of the project where I am trying to do the “using Plots”, so it case of not specified (no --project option), it is
"C:\Users\yourlogin\.julia\environments\v1.5/Manifest.jl"
Thanks again! I mark your answer as solution, awaiting more permanent fix as @giordano said in