Just use the share button, add them by username or email (possibly preferred) and choose Member (default, not Observer) and add them; only then can you go back (that's my experience; I don't know why that is) and change their role to Admin.
Note that's Board Admin, not Workspace.
Plus, that's assuming you're on Premium ( perhaps Standard?) not on Free, because as far as I know, everyone on a board on Free is part of the space and an admin. If your board is in your (now personal) Workspace and they are in their own business Workspace you need to take that into account.
Allow me to ask: if they're the (former) employer, why weren't they a member or an admin before or transfer the board while you were stille there? I can't quite picture that use case, sorry.
If it's more complicated than that, consider moving that board (unless you need it as well) to a new temp workspace they can then join, then add it to their own space and drop you?
Reading back, I'm guessing I'm either overcomplicating or you might need to paint the scene here some further. Hope you work it out.
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