Why can't I edit this?

The banner at the top of the budget says why: view access, section access, a locked bid, an archived budget or a lapsed subscription.

You can view this budget: you were shared as a viewer. Ask the owner for edit access.
You can edit sections …: you are a section editor. The rest of the budget is view only.
The bid is locked: bid numbers are frozen. Press Switch to Running to change the working budget; actuals stay editable. An owner can unlock the bid.
This budget is archived: unarchive it, or ask the owner to.
The workspace subscription has lapsed: every budget is read-only until the owner renews.
A greyed-out menu item means your role cannot do that; for example, only owners and admins can share or save templates.

Not what you were looking for? Write to hello@actualsbudget.com.