Welcome to the Community!

Intuit Link is a capable basic client portal but you just can't expect advance functionalities like you would for other portal you pay to subscribe.  It is a bit temperamental to use but most problems can be avoided so long as the user (accountants and clients alike) open it only in incognito mode and with only a supported browser.

As for why your client wasn't able to upload the prior year returns, my hunch is that the scan was not optimized and the file size is above the 30MB limit.  Under normal circumstances, that limit should be sufficient for most users.  Your client may like to adjust the scanner settings or, if not possible, email you the file, encrypted, instead.

https://proconnect.intuit.com/community/banking/help/intuit-link-taxpayer-upload-documents-to-send-t...

https://proconnect.intuit.com/community/help-articles/help/intuit-link-frequently-asked-questions/01...