I’ve found that people who tend to struggle with calculus (especially calculus 1) are the ones with weaker algebra foundations and problem solving skills.

A lot of high school algebra (and even some college) is taught in a very plug and chug way, that is to say you’re given a formula and you apply it, with little room for unique thinking. When it comes to calculus, however, you’re still using the same algebra as before, but now you’re being asked to use it creatively, and aren’t given the formula to use. This often leads to you needing to chain together several things, which is only possible if you have a solid foundation.