Should you take work you can't put in your portfolio?
A client assignment may pay well while leaving you with nothing you can show the next prospect.

Work you can't show can still be worth taking if the fee and the work itself justify the time. If you're counting on it to attract your next client, decide how you'll reach that client without using the project as an example.
What do you need the assignment to do for your business?
Name what you are relying on before you accept. You might need the income, want more practice with a type of assignment or hope to show potential customers what you can do. Judge each of those expectations against the work being offered.
In a hypothetical example, a self-employed facilitator wants to win more direct bookings for planning workshops. An agency offers a series of workshops for its own client. The facilitator would prepare and run the sessions, but the agency has said the client's identity and workshop material must stay private.
The work could still suit the facilitator's business. They can assess the fee against the preparation and delivery time and decide whether they want to do the sessions. If they were also counting on publishing the finished workshop as their first example, that part of the plan needs another way forward.
What can you show after the project?
Count on the use that has actually been agreed. Before relying on a case study, ask the client or agency what you would be able to share and get a clear answer. Being able to say that you delivered workshops and being able to display a client's material would give you different ways to explain your service.
In the hypothetical, the agency keeps its restriction. The facilitator therefore plans without that material. Changing a name or putting the work behind a password would still leave the question of permission unanswered. If the agreement is unclear, resolve that question before including the material in a public sample.

Look at how your recent clients actually found you, too. If they mostly came through introductions, you may want to ask what made the person introducing you think of your work. If prospects repeatedly ask to see a finished example, take that request into account when you decide how to use the time left around this assignment.
Where will your next example come from?
Make room for an example you can create independently if that is what your next sales conversation needs. The facilitator could design a short workshop around a fictional business situation and explain what a participant would work through. Label it as a sample, so a prospect can distinguish it from a workshop delivered for a client.

That sample is another piece of work. Before accepting the agency assignment, the facilitator puts its preparation alongside the paid workshops and the customer conversations already in the calendar. If all of it will not fit, they can change the timing, ask for fewer sessions or decide that the agency work should wait.
Use the same comparison for your own offer. What would the paid assignment leave you able to show, and what would you still need to make or explain afterwards? Include that work when you judge whether the client assignment fits. You can then accept for the benefits you expect it to provide and make a realistic plan for the next opportunity.
If the work you can sell now differs from the work you want to build, you can bring a real offer to business coaching. We can look at what it would pay, how it would use your time and what you would change before taking it.