Hi Peggy, as far as I know, only X-Cart v4.7.12 is PHP v7.4 ready. You could try replacing those 2 PHP files ^ from a stock v4.7.12 distribution (backup first).
reBOOT will be updated for v4.7.12 this coming week. reDUX is already up to date, which you should be able to download from your
order page.
I haven't been able to test with PHP v7.4
yet, shock! but will try do a local test with XAMMP or something as soon as poss.