The main problem with this is that the tax system is set up to prevent you from under-reporting your income. Over-reporting it is essentially trivial, e.g. two people who are in the relevant income range exchange favors (do each others' laundry etc.), or claim to have, and then actually report the transactions as income and get the credit.
But there's something else you can do here which is really neat. Stop using a complicated progressive rate structure, and instead eliminate the phase out entirely. Now instead of low income people having a nominal 0% tax rate but an effective 50% benefits phase out rate and high income people having a nominal 30% tax rate, you just use a flat 35% tax rate which implicitly has the benefits phase out built into the tax system. Which means you don't need any of this income reporting or annual tax returns or anything of the kind, the employer/seller just withholds the fixed tax rate and you're done, and everybody unconditionally gets the UBI to provide the effect of a progressive rate structure.
