Yes, you can use the tool for free but you won’t get anything useful in the free space after some time you will have to upgrade so get proper analytics of your website.
Likewise, you can do 10 searches a day with the free SEMRush subscription. You can also audit a single website with less than 100 pages with the free SEMRush subscription.
Semrush offer both free and paid service. If you uses free, can add only one website for audit and that for maximum 100 pages. In paid you can add more than 10 website for audit for more than 100 pages.