What is really meant by "Invalid B-tree node size"? How to solve it without expensive 3rd party softwares?
It basically means the file can't be looked in the B-Tree. A more in-depth explanation can be found at https://www.easeus.com/mac-file-recovery/invalid-b-tree-node-size.html
Option 1 - Disk Utility
It's an inbuilt disk repair utility that repairs most commonly seen hard drive problems.

Choose Apple menu, click Restart.
Press and hold the Command and R keys until the Apple logo appears and then release the keys.
Click Disk Utility and click Continue.
In the sidebar, select the disk with invalid b-tree node size and you want to repair, click the First Aid button.
Click Run.

If Disk Utility can't repair your disk, run it again. Or else, move on the next solutions.
Option 2 - Fsck command
The fsck command is used to repair disk and file system problems on your Mac.

Start your Mac in single-user mode. Restart it, and then press and hold the Command+S keys while it boots.
Type the command /sbin/fsck -fy into the terminal and press Enter to start a file system check.
When the fsck command has your disk checked and repaired, type reboot and press Enter. Your Mac will reboot and return to the usual login screen.

Option 3 - Reformat
Take the Mac hard drive reformatting as the last step, because this formatting process will erase all data on the drive completely. Make sure you've backed up your hard drive with Time Machine.

Launch Disk Utility. Click the icon for your external hard drive in the sidebar on the left.
Click the Erase tab along the top of the window.
From the Volume Format menu, choose Mac OS Extended (Journaled).
Enter a name for the external hard drive in the Name field.
Click the Erase button.

Disclaimer This answer is originally from https://www.easeus.com/mac-file-recovery/invalid-b-tree-node-size.html and was helpful for me when i had the same issue.