We have found that SAM.gov has recently flagged using Stable as the physical address because of their internal verification processes that can be variable from time to time.

As a workaround for now, we'd recommend using a residential address.
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For context, this is usually because they want their customers to share an address associated with the physical space where day to day operations occur as part of fulfilling their Know Your Customer requirements, and this is frustratingly an issue that all physical and virtual mailboxes run into.
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We've found that other forms of verification i.e a business registration doc, tax letter, etc can work as additional verification to open an account, if you have the option to submit any of that. In addition, we've found that sometimes the verification requirements are the most strict while opening an account with DUNS, so sometimes it may make sense to use the residential address and then update your address to Stable down the road. Worst case, you can use Stable as your mailing address so that we will receive any important documents that may get sent to you.