QB Issue Resolution:

If you are adding it on the fly, quick add, you will not be prompted for a number. If you otherwise are adding an account the number field will be displayed. Only if you enforce numbers, as opposed to just using them, you could not add an account without a number. But you can always edit an account after the fact, including adding or changing its number.

Resolution for Issue 'how to enter an account number when adding a new account to the chart of accounts' available: Yes (Solved).
Source: Intuit Community forum.

how to enter an account number when adding a new account to the chart of accounts: 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.

Support for account number
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Error Recovery for account number [Canada]
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