Whether a resume is required is up to the employer: some ask for one, some make it optional, and some don't ask at all. If you're prompted to upload one, you'll need a PDF file up to 5MB. If the step is optional, you can skip it and continue.
How to upload your resume
When you reach the resume step:
Click the upload area or Choose file.
Select your resume saved as a PDF (up to 5MB).
Wait for the confirmation that it uploaded, then continue.
If your resume is in another format, such as Word or Pages, export or "Save As" a PDF first, then upload that.
If the upload won't work
A few quick things to check if the file won't go through:
Confirm the file is a PDF and under 5MB. Large files with lots of images are the most common cause, so try exporting a lighter PDF.
Refresh the page and try again.
Try a different supported browser, such as the latest Chrome or Safari.
If you're on a phone, make sure the file is saved somewhere you can browse to (Files, Drive, or similar).
If you don't have a resume
If the employer made the resume optional, you can simply skip it. If it's required and you don't have one ready, a short one-page document is enough: list your most recent roles, dates, and a few responsibilities, then save it as a PDF. The employer is looking for a quick picture of your background, not a polished design.