“I Could Do with a Coffee.” Means “I’d Like One”

“I could do with a coffee” means that a coffee would be welcome or useful right now. It does not describe an ability to “do” something with coffee. The phrase could do with expresses a need or desire, usually in a gentler, more conversational way than I need.

After a long walk, you might say, “I could do with a coffee. How about you?” It can signal a real need or simply introduce a relaxed suggestion. Adding really makes it stronger: “I could really do with a break.” In American English, “I could use a coffee” is more common and has a similar meaning.

https://dictionary.cambridge.org/dictionary/english/could-do-with
https://www.collinsdictionary.com/dictionary/english/could-do-with