Does anyone know why I'm having this error?

[LunarCatUSA]

[Hoeloe]

Are you trying to load a level made in 38A? You can't do that. You'd need to convert it to a lvlx file first.

[LunarCatUSA]

Are you trying to load a level made in 38A? You can't do that. You'd need to convert it to a lvlx file first.

So do I just rename the extension at the end to lvlx? Or is there more to it than that?

[Hoeloe]

You need to re-save the file as a .lvlx file from PGE. Just renaming the extension doesn't change the file format, it only changes how your operating system will try to read it. Since the 38A level format is very different from the PGE .lvlx format, if you just rename the extension, it'll be unable to read it at all.

[LunarCatUSA]

Hmm... it also may be due to the fact I have a gravitational field present and that only existed in SMBX38A, so I gotta get rid of that. (And try to find another way to magnetize the player into a death zone if they wander in a part they’re not supposed to be in)

Added in 1 minute 23 seconds:
Yup, that was it!

Added in 1 minute 4 seconds:
Now I gotta change that through out all my levels. (Gotta edit it out a velocity increase field in a racing level and give the player a new way to speed up for that, but at least I found out what it was)