When I want to create a deposit the customer does not exist but when I want to create the new customer I can't because it already exits???

There are several factors we need to consider to help us determine why the recent performance of QuickBooks Online (QBO) is slow on your end, sara109.

While QuickBooks is web-based, it depends on your device's processing power, available memory (RAM), and Internet bandwidth. Any shortfall in these areas can impact operations. 

To improve the system's performance and ensure you're getting the most out of your QuickBooks experience, check out the following recommendations:

• Use the Google Chrome browser. If you're already doing so, make sure you're utilizing the latest release by going to the Chrome menu, selecting Help, and then About Google Chrome.
• Confirm that your operating system is up to date and you meet or exceed the System Requirements for QuickBooks Online.
• Verify your internet speed (bandwidth) is not limiting your QuickBooks Online experience. Run a Google Speed Test to confirm your bandwidth and connection speed.

For more insight, please see this article: Why is my QuickBooks Online slow?

On the other hand, this matter could also be caused by your browser's cached files. QuickBooks relies on the cache to function efficiently, so clearing any outdated ones can help fix the customer list issue you're experiencing and improve the overall system performance.

Click any of the links below for detailed step-by-step instructions tailored to different browsers:

• Firefox
• Google Chrome
• Safari for Mac
• Safari for iPad and iPhone
• Microsoft Edge

If this issue continues, I recommend clicking the browser's Refresh icon (please see the screenshot below) or using other supported browsers. 

If you have further questions, let us know in the comments.