Where do I find a cusip number frim my brokerage account to complete my return ?

Box 14 on a 1099-INT?

.....you only enter it if you only owned one particular Muni bond, with box 8 $$ showing....and you would get that from your own access to your Online account, and your listing of what you hold for that brokerage.

For multiple Muni bonds you own....you'd just leave box 14 empty

I always leave box 14 on mine empty, and it works entirely OK.

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If leaving box 14 empty doesn't work.

Then

...delete that 1099-INT and enter it manually  (Again, leaving box 14 empty)  don't put any zero (0.0) values in any of the 1099-INT boxes either.....if some box has a zero value, leave that box empty.