You can still fast on the 10th of Muharram الــــحـــمـــد للّٰـــــه
Those who did not Fast on the 9th of Muharram can still Fast on the 10th.
Fasting on the 9th of Muharram is preferred and not a condition.
Fasting on the 9th & 10th of Muharram is in order to oppose the Yahood who only Fast on the 10th.
If one only Fasts on the 10th of Muharram, then their Fasting is valid. Those who Fast on the 10th of Muharram will have reward of having the sins of their previous year forgiven.
One should not oppose the Sunnah if they have the ability.
One should be keen to emulate the Prophet ﷺ to the best of their ability.
So, intentionally missing out on the 9th is not encouraged. But those who missed on the 9th can still Fast on the 10th.
To oppose the Yahood, one can Fast on the 10th & 11th of Muharram (as some of the scholars have said), but the preferred is to Fast on the 9th & 10th of Muharram, as the Prophet ﷺ had preferred.
For he ﷺ wanted to excel the Jews by Fasting a day ahead of them, and joining it with the 10th of Muharram.
Fasting on all the three days together, i.e. the 9th, 10th, and 11th is not supported by an authentic Hadeeth, rather, the Hadeeth in this regards is weak.
حَدَّثَنِي يَعْقُوبُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا رَوْحٌ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا أَبُو بِشْرٍ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ لَمَّا قَدِمَ رَسُولُ اللَّهِ صلى الله عليه وسلم الْمَدِينَةَ، وَالْيَهُودُ تَصُومُ عَاشُورَاءَ، فَسَأَلَهُمْ، فَقَالُوا هَذَا الْيَوْمُ الَّذِي ظَهَرَ فِيهِ مُوسَى عَلَى فِرْعَوْنَ، فَقَالَ النَّبِيُّ صلى الله عليه وسلم “ نَحْنُ أَوْلَى بِمُوسَى مِنْهُمْ فَصُومُوهُ ”.
Narrated Ibn `Abbas: When Allah’s Messenger (ﷺ) arrived at Medina, he found the Jews observing the fast on the day of ‘Ashura’ (10th of Muharram). The Prophet (ﷺ) asked them (about it) and they replied, “This is the day when Moses became victorious over Pharaoh.” The Prophet (ﷺ) said (to the Muslims), “We are nearer to Moses than they, so fast on this day.”
