You entered an assignment agreement with the client. If you were not able to cancel your shift before the cancellation term, it’s your own responsibility to find a suitable substitute. The client is expecting someone that is on the same level as you are.
In the case where you don’t succeed in finding a suitable substitute, your client has the possibility to issue a no-show. A no-show lowers your attendance percentage, which is visible for clients when you apply for a shift.
Informing your client upfront might prevent this, but you will also have a chance to dispute the fee after the shift. You will have a one-time option to come with a counterproposal in which you share your reasoning for not making it to your shift. It’s still the client that makes the final decision whether to maintain or waive the no-show.
Pay attention: If you cancel a shift before the cancellation term goes into effect, you won’t be expected to find a suitable substitute.