Before you can enter any starting balances you need to go Settings - Start Date and enter a date.
Then under the Cash Accounts tab - click “New Cash Account” and as part of the set up you can enter starting balances. For other Asset & Liability accounts go to Settings - Chart of Accounts.
For Customers & Suppliers read below
https://forum.manager.io/t/setting-up-starting-balances-for-customers/5494
https://forum.manager.io/t/setting-up-starting-balances-for-suppliers/7227
For inventory
https://forum.manager.io/t/setting-up-starting-balances-for-inventory-items/5497