Supermetrics fetches SEMRush data from SEMRush API. Each API call has a cost, and the cost of requests is measured in API units. Units are determined by the type of report or request retrieved from SEMRush API, and fetching historical data requires more units than live data. One dollar is equal to approximately 20,000 API units. See details on API call unit requirements for SEMRush Analytics API here, and SEMRush Projects API here.
To purchase API units, log in to SEMRush, click on your profile icon on top right and select "Subscription info". On this page you'll find a tab called "API units", with the option to purchase more units.