What’s the issue with saying the extend?

What kind of error is it?

This error is a misspelling. "The extend" incorrectly uses "extend" instead of "extent", likely involving a mix-up between the noun and verb forms that sound similar.

Why do people make this mistake?

People often confuse "extend" and "extent" because they sound similar and are related in meaning, albeit differently as a verb and noun. The mistake happens when people attempt to use a noun indicating the degree or range of something, but mistakenly use the verb form "extend", which means to enlarge or expand.

What is correct?

The correct word is "extent", which is a noun used to describe the degree, range, or scope of something. "Extend" is a verb and does not fit grammatically in this context.