Ah it's because the variable you're substituting doesn't exist all the time! You can use `format` or fstrings to fix this, for example:
add AlphaMask("menu/memories_{}.png".format(replaydate[replayCurrent]),
The problem is that your `$ currentreplayDate = replaydate[replayCurrent]` line only runs when the screen is up, so most of the time that variable doesn't exist and Ren'Py doesn't know what to search for in the dynamic image. Using fstrings or format substitutes the variable right in the path instead, so it isn't trying to search for an incomplete image path.