How to alter a flashback data archive that needs additional quota

SELECT F.FLASHBACK_ARCHIVE_NAME, F.TABLESPACE_NAME,F.QUOTA_IN_MB, 
       (SELECT ROUND(SUM(S.BYTES)/1024/1024/1024)
    FROM DBA_SEGMENTS S 
    WHERE S.TABLESPACE_NAME=F.TABLESPACE_NAME) "occupied"
 FROM  DBA_FLASHBACK_ARCHIVE_TS F;

FLASHBACK_ARCHIVE_NAME	TABLESPACE_NAME	QUOTA_IN_MB	occupied
MY_FDA	FDA	20480	19

If you've reached the quota, add a larger one, like this:

ALTER FLASHBACK ARCHIVE MY_FDA 
MODIFY TABLESPACE FDA QUOTA 20 G;