Is it Costomise option disable?

Glad to see you here in the Community, @innotech-ap. Let me further assist you so you’ll be able to customize your estimate custom template in QuickBooks Online.

You can set an estimate custom template as default which can make them unavailable on your end. If you want to customise the template without affecting the invoice format, here’s how to create an estimate from scratch:

1. From QuickBooks Online, click the Gear icon > Custom Form Styles.
2. On the New Style drop-down menu, select Estimate.
3. Use the settings on each tab to build your estimate from the ground up. Be sure to click Save before finishing.
   
   

For your reference, check out this article: Customise invoices, estimates, and cash memos in QuickBooks Online.

Just in case you want to import custom form styles for estimates, please see this link for further details.

I'll be glad to help you if you have any other questions about customising sales forms in QBO.​ Have a good day!