They can, or you can come some other time, when you can and ask them to process your biometrics and they will do.
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You may also, in an abundance of caution, mail a brief requests for your needed change (to the address indicated at the bottom of the I-797. It is also always a good idea to send the letter via certified mail or at least with delivery confirmation, and of course keep a copy of the letter and I-797 for your records. If you can go before your scheduled time and date (avoid Mondays and Fridays), especially with your condition, you would likely be allowed to have your fingerprints taken that very day.