Can I use the music in work for clients?

Yes, on the Business and Business Pro plans, or for a single video with a Business / Advertising single-track license.

Your client can be a company of any size. The client may use the production you made for them, as you delivered it, within the uses your license permits. The client does not get a license to the music itself.

The Individual plan and the Online / Social single-track license are for your own channels only, not for client work.