Error: At least one of the variations associated with this listing must have a quantity greater than 0. Error: To require immediate payment, you must specify a Buy It Now price.

If you see this error when trying to OS(out-of-stock) a BIN GTC multi-variation listing, then you might have disabled the OS(out-of-stock) function manually by accident.

This function is enabled by default on eBay backend and also in SoldEazy.

Resolution:

Go to Channel > Sales Channel, find the account of your listing, click the gear button on the right and enable the OS(out-of-stock) function as below.