$ mkdir /tmp/test
$ /usr/local/opt/nss/bin/modutil -fips true -dbdir /tmp/test

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A PKCS #11 module returned CKR_DEVICE_ERROR, indicating that a problem has occurred with the token or slot.
ERROR: Unable to switch FIPS modes.
Function                     # Calls         Time         Avg.     % Time

C_Initialize                       1       1000us    1000.00us    100.00%
C_Finalize                         1          0 z       0.00us      0.00%
C_GetInfo                          1          0 z       0.00us      0.00%
C_GetSlotList                      2          0 z       0.00us      0.00%
C_GetSlotInfo                      2          0 z       0.00us      0.00%
C_GetTokenInfo                     2          0 z       0.00us      0.00%
C_GetMechanismList                 4          0 z       0.00us      0.00%
C_OpenSession                      3          0 z       0.00us      0.00%
C_CloseSession                     1          0 z       0.00us      0.00%
C_CloseAllSessions                 2          0 z       0.00us      0.00%
C_GetSessionInfo                   1          0 z       0.00us      0.00%
C_FindObjectsInit                  2          0 z       0.00us      0.00%
C_FindObjects                      2          0 z       0.00us      0.00%
C_FindObjectsFinal                 2          0 z       0.00us      0.00%

                   Totals         26       1000us


Maximum number of concurrent open sessions: 3