You may get this error during:
target public folder mailbox quota is exceeded,
target public folder quota is exceeded.
The above error will appear when your target primary public folder mailbox exceeds its maximum allowed size, 50 GB or 100 GB, depending on your Office 365 plan.
To solve this problem, you need to wait for up to two weeks for the Office 365 AutoSplit feature to kick in. Automatic splitting will create a secondary mailbox for public folders and move some data from the primary mailbox to the secondary mailbox.
You may also get the error if your source and target public folder quotas do not match, for example, when migrating a public folder that exceeds the maximum folder size allowed in your target environment.