し: adding reasons and characteristics
See how し builds an evaluation from several points, and compare it with たり for examples of activities.
Give one point while leaving room for more
し presents a characteristic or reason as one among several. この靴は軽いし、歩きやすいです。 means that the shoes are light and easy to walk in, combining two advantages in an evaluation.
The connection does not assert that the first advantage caused the second. It places being light and being easy to walk in alongside each other, so do not automatically translate the first clause as a direct cause whenever you see し.
Keep the clause-ending form before し
In plain-style clauses, use forms such as 安いし, 便利だし, and 学生だし. Positive present forms of nouns and な-adjectives keep だ; do not use the noun-modifying form to produce 便利なし.
時間もないし、今日はやめましょう。 means that, since there is no time either, we should leave it for today. A single し can leave room for other circumstances; speakers do not have to state two reasons aloud.
たり changes the focus
休みの日は本を読んだり、散歩したりします。 gives examples of activities on days off: reading books and taking walks. たり can list representative activities this way without itself presenting them as advantages supporting a choice.
この町は海が近いし、空気もきれいです。 adds together features of a town: it is close to the sea, and the air is clean. Start by distinguishing し for accumulated characteristics or reasons from たり for examples of activities.
時間もないし、今日はやめましょう。 contains only one し. Is the sentence wrong because only one reason is stated?
Show the answer and why
No. It gives the lack of time as one consideration while leaving room for others. し does not have to appear twice.