How to establish a relationship between tables?

Question

SQL

Hello!
There are 2 tables:

User table

Order table

It is necessary to establish a relationship from the Client and Manager fields from the Orders table to the Login (by task) or Name (if not to Login) field from the Users table.
When trying to establish a relationship:

The following error appears:

I don’t understand what to do, I googled, but I didn’t find a good answer - I googled badly, but I still don’t know how to google so well -_-.
Please tell me what am I doing wrong?
PS Thank you all so much!

Answer 1

Actually, everything is written in the error, the Users.Login column is neither PK nor UNIQUE. Actually, make it so.

Answer 2

The Password field is not a key.