Most likely because you have the “To Be Printed” box marked.
Uncheck that and you will be able to enter the cheque number.
Resolution for 'box marked issue: 'Enter write cheque number' available: Yes.
The box marked 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 box marked issue yourself by looking for a resolution described in the article. If it is a complex box marked issue or you are unable to solve the issue, you may contact us by clicking here or by using other box marked support options.
For more information about resolving issues related to box marked in Quickbooks/QBO or to get support for the issue 'Enter write cheque number', click the link below: