USF is a Supply Mains UnderV error.

The softstart will display 'USF' fault when you have a run command but there is low or no line power present on ATS.

On systems that use an input contactor (also known as an Isolation Contactor) this can occur if the contactor does not close within 200 ms of the start signal.
Check the programming of the R1 relay (normally set to Isolation when used in this manner) and verify the operation of the contactor and any interposing relays.

Newer ATS units (ATS480, 490 and 430) have some adjustment to the undervoltage threshold and UV detection delay that can be reviewed to help ride through power dips while in a running state.

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