Learn Spanish (Spain): Leo la página veintiuno.

In Spanish, numbers from 21 to 29 are always written as a single combined word. To do this, the final e in veinte changes to an i, and you attach the single digit right to it. That is why 21 is veintiuno, 22 is veintidós, and so on. For numbers 31 and higher, you will use separate words, like treinta y uno (thirty-one).

Since página is feminine, shouldn't it be la página veintiuna?

When a number is used as a label or an identifier (like a page number, a hotel room number, or a year), it stays in its default masculine form. You only change it to the feminine veintiuna when you are counting a quantity of feminine things, like saying Leo veintiuna páginas (I read twenty-one pages). Because this is "page number twenty-one", it stays veintiuno.

Why doesn't the sentence start with yo for "I"?

In Spanish, the ending of the verb usually tells you who is doing the action. The -o ending in leo already means "I read". You can include yo to say Yo leo if you want to emphasize that you are the one reading, but it is completely standard to leave it out.

Why does página have an accent mark?

The written accent mark tells you to put the spoken emphasis on that specific syllable: PA-gi-na. Without it, standard Spanish reading rules would tell you to stress the second-to-last syllable (pa-GI-na). The mark overrides that rule so you know exactly how to say it.