Self-employed home improvement business - Where do I enter the expense for a storage unit used for storing job materials and tools?

Is it a unit you rent?  Or built?  You can enter rent on Schedule C .

How to enter self employment expenses

https://ttlc.intuit.com/community/entering-importing/help/where-do-i-enter-my-self-employment-business-expenses-like-home-office-vehicle-mileage-and-supplies/00/27109

Assuming you are reporting business income/expenses on SCH C, and further assuming you are renting this storage unit, in Business Expenses/Other Common Business Expenses there is a section specifically titled "Rental Expenses".

If you purchased this storage unit and you own it, then it's entered as a business asset in the Business Assets section and depreciated over time.