QB Issue Resolution:

To prevent other users from amending the transactions, you can close the books, then set the closing date to March 18. Closing the books helps protect the transactions in previous accounting periods from being changed.

Here’s how to close the books:

  1.  Click the Gear icon.
  2. Choose Account and Settings.
  3. Go to the Advanced tab.
  4. Click the Pencil icon in the Accounting row.
  5. Put a check mark in the Close the books box.
  6. Add 18/03/2018 as the Closing date, then click Save.
  7. Click Done.

I’ve got an article for future reference: Close the books.

Resolution for Issue 'We had done audit for the period until Mar’18, can we lock the period from other user to amend ?' available: Yes (Solved).
Source: Intuit Community forum.

We had done audit for the period until Mar’18, can we lock the period from other user to amend ?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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