assuming this is a publicly traded (PTP) or master limited (MLP) partnership traded on a stock exchange the cover letter may contain language like the following. "The Partnership will report items of international tax relevance to our partners on Schedule K-3. All of our income is effectively connected with a United States trade or business. A limited number of unitholders (primarily foreign unitholders, unitholders computing a foreign tax credit on their tax return and certain corporate and/or partnership unitholders) may need the detailed information disclosed on Schedule K-3 for their specific reporting requirements. We do not expect to report either (1) any foreign income taxes paid or accrued on Schedule K-1, Line 21 or (2) any foreign-derived intangible income. in other words if you are a US citizen, there will be nothing to report if line 21 does not have an amount so you don't check box 16 on the Turbiotax k-1. if there is an amount on line 21 you can uncheck box 16 to amend and be able to e-file but then you'll have to leave line 21 blank and amend a second time to get the foreign tax credit if you want it.
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as for when to file an amended return you must wait until the original is fully processed then if it will result in a refund you have until 4/15/2026 to file. if you owe there is no deadline to file but filing after 4/18/2023 will result in penalties and interest for late payment.
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look in box 20 or any supplemental schedule for an explanation of 13W - only the partnership can tell you what it's for
