Unless you requested a filing extension by April 15, you cannot make a solo 401(k) contribution for 2018 after April 15, 2019.
Assuming that you did not request a filing extension, you must amend your 2018 tax return to remove the self-employed retirement contribution that you did not make and pay the additional taxes that result from the removal of this deduction.
There can also be other potential side effects that result from the increase in AGI. In particular, if you made any Roth IRA contributions, make sure that the contributions have not become excess contributions.
