That prefix isn’t standard for USCIS, which might be causing the issue. Double-check the receipt number on your form I-797C to make sure it’s entered correctly (no spaces or dashes). If it still doesn’t work try using a different browser or an incognito window. Sometimes the USCIS website has compatibility issues. If nothing works, call the USCIS contact center. They can confirm your receipt number and may even schedule an in-person appointment if needed. Hope this helps. My response was based on my research, knowledge and external sources like skiplegal.ai

@March 2022 : Did you mean "ZSF"? For USCIS receipts, the "Z" indicates asylum case and the other two numbers refer to the asylum office where your case was originally received. To my knowledge, there is no asylum office with DF, unless they added one that I don't know about. However, there's one with SF, and that's San Francisco. And it's a very slow office usually. Check your receipt, and the address on it. That will tell you where your case is. Then, call that office to ask about your number and how to check the status of your case. Another thing that could work provided you have a valid case number, you can check it here within Legally. Sometimes the USCIS website won't recognize/accept some case numbers (I don't know why). But here on Legally, the system would and you'd get a timeline. I'm speaking/writing from experience.