Hi! に with the original verb 乗る indicates the object you ride "bicycle, car, train, etc." by itself, it means (in) or (on) like (ride in a car or ride on a bicycle). But first, I'd like to emphasize that this original verb 乗る takes the particles が and に, where が indicates the subject or the rider, and に indicates the object or the thing you'll be riding. The expression "can do something" can be expressed in two forms. Depending on the form you use, the particles of the original verb either remain the same, or change. So, "can ride a bicycle" can be expressed in two ways: 1. (私が)自転車に乗ることができる。 2. (私に)自転車が乗れる。 When using the first form. The particles remain the same because the verb form remained the same "乗る". But as you can see in the second expression, the verb form changes. This is called the potential form of a verb. Its form forces the nouns in the sentence to take specific particles. The pattern is: (person) に (thing) が (verb in potential form). Now, to express (cannot ride) as in your example, you can say: 自転車に乗ることが出来ないので・・・ Or 自転車が乗れないので・・・ If you use the second expression, like the one in your question, it's preferable to use が instead of に just to follow the rule of using the potential form of a verb. But if you want to use に, use the first expression. Hope that helps.

*** に ： a marker for "place" - 東京に住んでいます。 - アメリカにいきます。 - そんなにいじめないでください。 **** 乗る : get on - 自転車に乗る　（* get on, on the bycicle) - 飛行機に乗る （* get on, on the plane) - 戦車に乗る （* get on, on the tank) - 電車に乗る （* get on, on the train) - バイクに乗る （* get on, on the bike) ***　乗れる 乗れる（NORERU) ＝　乗り（NORI：連体形 of 乗る)＋得る（ERU: get, obtain) ※ 行ける（IKERU）＝　行き（IKI)＋得る（ERU) 書ける（KAKERU)＝書き（KAI)＋得る（ERU) 例文： 「誰か、この中で自転車に乗れる人、いますか？」 Can anybody here ride a bicycle? 乗れる ＝ 乗り＋得る - 自転車に乗れる 出来る ＝ 出る＆来る - 英語が出来る　：English (naturally) comes out of me → can do:(speak/read/write) English.

In the case, I can't ride on a bicycle, "に" is used to express "on".