I do not see Inventory in the sidebar.

Switch on Inventory & supply chain in Settings > Features. Stock tracking also needs a plan that includes inventory.

A sale did not take stock off.

Stock comes off when an order is completed, at the location of the sale. Check the movement log and that you are looking at the right location.

With stock tracking on, every completed sale takes stock off and every refund puts it back. You set the starting count and Timeo keeps it up to date.

Open Settings > Features and switch on Inventory & supply chain. The Inventory menu appears in the sidebar (for gyms, its pages sit under Sell).
Open Stock under Inventory. Every active variant is listed with On hand, Reserved, Available and Threshold.
Press Adjust on a variant and choose Stock in.
Type the Quantity, pick a Reason such as Initial stock, and add a Unit cost if you know it.
Press Record stock in.
In the same sheet, type a Low-stock threshold and press Save threshold. When available stock falls to that number, the item shows as Low.

Stock out records waste or items taken out, and Adjustment corrects the count up or down.
Transfer moves stock to another location: pick the Destination location and press Transfer stock.
Set cost overwrites the average unit cost without changing quantities.
Low stock only filters the list to items that need reordering.
Inventory movements lists every change: sales, refunds, received orders, transfers and manual adjustments, with who made them.

Nothing changes for customers. Staff see stock levels and low-stock items in the console.

I do not see Inventory in the sidebar.: Switch on Inventory & supply chain in Settings > Features. Stock tracking also needs a plan that includes inventory.
A sale did not take stock off.: Stock comes off when an order is completed, at the location of the sale. Check the movement log and that you are looking at the right location.