How come he couldn’t open the attic from the inside?

THAT IS NOT A PLOT HOLE...RATHER A PLOT CONVENIENCE. NOEMOJI

Because it's a silly comedy. If he could have opened the hatch from the inside, there would have been no scene with him trapped in the attic. What a more skilled film maker, like Buster Keaton or Harold Lloyd, would have done would be to think of some reason the hatch got stuck or locked, so it would have made more sense.