Excel is indispensable for startups. Excel is not just for financial modelling and projections. It’s essential for ideation and brainstorming business model scenarios. If you are not already comfortable and proficient with Excel, you’d want to get up-to-speed quickly.
There is no shortage of Excel courses out there for all levels. You’d need to have a good grasp of the basics. Many beginner level courses are painful and tedious to get through. This one from HunKim.com which is free on YouTube has a good balance between good basics coverage without the “for dummies” pitch.