"いっぱんてき could be an inflection of いっぱん, with these forms:
Te-form. It is a connective form of the verb."
but it is clearly not the case. I checked the entry and it is does not seem to be a problem with the entry as いっぱん appears as a noun there. I assume that this message is an automatic guess of jisho.org just as the verb inflections feature.
Somehow jisho.org interprets いっぱん as a verb.
The search for いっぱんてき ( https://jisho.org/search/いっぱんてき ) shows a message as though いっぱん was a verb,
"いっぱんてき could be an inflection of いっぱん, with these forms:
Te-form. It is a connective form of the verb."
but it is clearly not the case. I checked the entry and it is does not seem to be a problem with the entry as いっぱん appears as a noun there. I assume that this message is an automatic guess of jisho.org just as the verb inflections feature.