How do i enter cheques with numbers lower than the automatically generated numbers?

Welcome to the Community, boydcolleenm. I've some information you need about cheque numbers.

If you mean to modify the Cheque no., those were automatically generated from the system. That said, we can't enter numbers lower than what is shown. 

If you change it, a prompt message will appear. 

If this isn't the case, have you encountered errors besides the prompt message above? This way, I can provide you with a proper resolution. 

Here's an article you can refer to for more details on writing, entering cheques, and printing them: Create and record cheques in QuickBooks Online. 

For more tips on what transaction to use in future tasks, please visit this reference: Learn the difference between bills, cheques, and expenses in QuickBooks Online. 

Drop by this thread again if you have future concerns with QuickBooks Online. I'm here to offer help.