While prepared tax return showing " Preparing error check...." for a while, unusually how long it takes to complete error check?

Normally it only takes a few seconds to perform error check using the online editions, 5 to 10 seconds.

If you are seeing the error check process continue for an inordinate amount of time, then Sign Off your online account by clicking on Sign Out at the bottom left corner of the online program screen.

Close the TurboTax window and then clear cache on your web browser, see this TurboTax support FAQ - https://ttlc.intuit.com/turbotax-support/en-us/help-article/product-system-requirements/clear-cache/L8EEqjh3R_US_en_US?uid=loblw3qm

Sign back onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

See if the problem persists.  If it does, change your web browser to one of these that you are not already using, Google Chrome, Mozilla Firefox, Microsoft Edge.