I have it working in PS 8.1.7 running PHP 8.1.29.
It was tricky to figure out, but with a little digging through all these posts I figured it out. If you are trying to edit the zip file as mentioned in a previous post and it still fails, you can try it like this. It worked for me.
Before getting into the steps, this is a fresh install of PS 8.1.7. No additional modules installed other than Printful so the trouble shooting could be directed at the only thing that could be wrong.
Here is what (as of this time and date) needs to be done.
1. Get the latest zip file from Printful, but do not install it.
2. You will need to edit a file in the zip file WITHOUT unpacking it first. Get 7-Zip to do this, it is what I used ( https://www.7-zip.org/ ).
3. Open 7-Zip, and from within it, open the zip file, locate the printful.php file and open it in notepad. Go to around line 85. You are looking for this code 'max' => '8.1.3',
4. Change that to 8.1.7 or whatever version you are using and save the file. THEN CLOSE NOTEPAD. This is essential, because until you close notepad, 7-Zip will not update the file for you.
5. Once notepad is closed, you should get a dialog box that says something to the effect of the file has changed would you like to save it. Select yes or ok if it does. Then CLOSE 7-ZIp.
6. Now double check the file inside your zip file by opening 7-zip again and open the printful.php again the same way you did before. Check the line of code you edited and make sure the change is still there. If it is, you should be able to install the module by using the standard upload through Back Office.
I hope this helps. I spent a full day messing with this and after reading some of these posts I finally got this to work. To be sure, I have added products to the shop from Printful without getting the red/pink error as shown in previous posts (I was getting those before on an "almost" got it install), but now it is working as it should.
Cheers.