What does the slang word "can" mean?

Solution 1:

The first two mean toilet or bathroom.

The noun can also means

a metal or plastic container for holding film on cores or reels.

When filming is finished, the film will be put into the can. Therefore, in the can means completed.

[Dictionary.Reference.Com]

In the can:

Completed, as in about a hundred pages of her next book are in the can. This usage originated in filmmaking to describe a completed motion picture, when film was literally put into a can or canister. [Slang; c. 1930]

[The American Heritage® Dictionary of Idioms]

Your example:

Finally, our planning is completed and we can begin construction tomorrow.

Solution 2:

The first two examples do mean toilet.

The third example is a totally different meaning. The idiom is actually in the can

• (of a film, piece of music, etc) having been recorded, processed, edited, etc
• (informal) arranged or agreed the contract is almost in the can

Collins

While I do not have a reference, I suspect the latter sense is derived from the former.

Solution 3:

"In the can"--a term for an entire film or a subset of shots that are all finished shooting; also denotes when a director has the take that he wanted.

It refers to any project or plan that is ready to go. Think canned fruit or veggies...