Lets try a step-by-step guide...

Please follow guide and spool off eveything you do - in case something happens we will appreciate to see whole session.

1- Drop MAM_ASSETS table
2- Create MAM_ASSETS - be sure no column has NOT NULL option
3- desc MAM_ASSETS
4- import MAM_ASSETS with options... ignore=y indexes=n constraints=n
5- desc MAM_ASSETS