QB Issue Resolution:

We tried entering the same HSN code in my file at our end and it got saved without any error. Request you to switch the browser and try again. if you still face the issue, request you to contact our support team. https://help.quickbooks.intuit.com/en_IN/contact

Resolution for Issue 'HSN code inputting error message – please enter a valid code even if am putting a exact code. Am fed up …. how' available: Yes (Solved).
Source: Intuit Community forum.

HSN code inputting error message – please enter a valid code even if am putting a exact code. Am fed up …. how: 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 exact code
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Error Recovery for exact code [Canada]
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