I cannot get my old macbook late 2009 running macOS Sierra 10.12.6 to transfer information to my new macbook air using migration assistant. Is there something I'm doing wrong?

There are a few ways to move files from older to newer Mac.

Migration 'assistant' during first-run of newer Mac; or later on

once its already ran. Or others I've read about, but not tried.

Manual migration is preferred by some users; to pick & choose.

• How To Perform a Manual Mac System Migration [MacRx] - cultofmac

https://www.cultofmac.com/45114/how-to-perform-a-manual-mac-system-migration-macrx/

• How to move your content to a new Mac - Apple Support

https://support.apple.com/en-us/HT204350

• Transfer your data to your new iMac - Apple Support

https://support.apple.com/guide/imac/transfer-your-data-apd6a937e818/mac

Suppose you followed migration assistant, then needed to resume?

• Moving to a New Mac: What's Left to Do After Migration | tidbits

https://tidbits.com/2020/09/07/moving-to-a-new-mac-whats-left-to-do-after-migration/

You could also restore or use saved older content from Time Machine.

There's more than one way; different support links. By Apple & others.

And too, using Clones of boot-capable systems; to selectively reuse.

Good luck & happy computing!🌻|🐝

i had no problem with migration until Sierra

but that was the software manufacture who input expiration dates on their applications.

anyways, hope this was reassuring.