You cannot report the gain in a different year from the year you receive it.
There is no duty to amend a tax return if you made an honest mistake (even if the mistake results in more tax owed). However, you may wish to file an amended return, either to get a refund or to stop interest/penalties from accruing. You might want to enter the cap gain and see if it makes a difference. It might not result in any extra tax (some people are in the zero percent cap gain bracket).
More detail here:
https://www.thetaxadviser.com/issues/2017/jan/amed-return-client-decision.html
"The Supreme Court held that, despite these and other references to amended returns in the Treasury regulations, "[n]one of these provisions, however, requires the filing of [an amended return]" (Badaraccoat 397). Similarly, the Tax Court held that taxpayers are not required by statute to file an amended return when an originally filed tax return is discovered to be incorrect and that the failure to file an amended return in this context does not, by itself, establish an intent to evade tax (Broadhead, T.C. Memo.1955-328)."
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