First, ask your client if they'd be willing to make an exception for portfolio use only. A lot of clients will say yes if you ask nicely and make it clear it's just for your portfolio — not for public promotion. If they say no, here's what you can do instead:
→ Redact it — this means removing all identifying information, not just the client's name. Think: brand names, product names, industry-specific language, any stats that could identify the company, logos, and screenshots with visible branding. If someone could figure out who the client is from context clues, keep editing.
→ Recreate it — use the NDA'd project as inspiration and create a similar piece from scratch for a fictional client. You can speak to your process without sharing the actual work.
→ Use a testimonial instead — if the client is happy with your work but the work itself is off limits, ask if they'd be willing to write you a short testimonial. Social proof goes a long way.