This is 4.5 DOS?
I have had this error a long time ago. Can't remember exactly, but it may have something to do with the FILES= and BUFFERS= in the config.sys.
Paradox requires FILES=40 as a minimum (max value = 254, I think), but I usually have both set to 40.
Alertatively, it could just be that the table is damaged and you can use TUtility to fix it (that has appeared here before).
Phil