I have a 2001 Bonneville with about 125,000 miles I just bought it from someone last week and it started slipping when I drove it for about 30 mins I also got the code p1811 how do I fix this no engine lights or anything only other code is p0440 I just really need some help can this be pressure control solenoid or do I need new transmission or ecm

Evan Welcome.
The P0440 could be just the gas cap.
The P1811 (long adapt shift) code is symptomatic of a failing pressure solenoid, which is difficult to access at valve body.
In my case I had hard shifts and and used 3/4 can of seafoam transtune. It will help to clean the valve body and accumualtor.
I noticed an improvement in shifts fairly quickly with the transtune.
Next was installing a new trans filter & installing a TransGo 4T65E shift kit into the accumulator. 45k miles later all is well. Use Dex 6.
Here are 2 related threads with images.https://www.gmforum.com/buick-172/sh...-307429/page2/
https://www.gmforum.com/buick-172/transmission-shifting-hard-ocassionally-305902/

I'd bet the seller had the issue with the transmission then poured some trans-fix in it before selling.