You cannot have negative taxes paid ... did the foreign government pay you for making income ??? Ask schwab what this means ... maybe you got a refund of foreign taxes paid in a prior tax year giving you a negative entry.
I would just ignore it. but if you want more info look at the 1099 that Schwab sent you or you can download it from your account. is it the latest one (corrected) because frequently Schwab will issue revised forms? one year I didn't get the last revision until November of the following year. about 2 months after the extended due date of the 1040. on the 1099 index, under Supplemental info, there will be a page that lists FTC by stock and lists the foreign income for that stock. then you can call Schwab for further details. I have to say that in over 50 years of tax preparation for clients I have never seen a 1099 with negative FTC.