Cannot install Office 365 ProPlus on more than one device

Thanks for the additional information, that licence looks to have a restriction of only 1 install of Office, which matches the screenshots and the error that was originally mentioned. 

This documentation actually alludes to having 5 installs available but perhaps there are different versions that vary in this way. This difference could be to do with the student versus faculty and staff options but if it is, I can't see it mentioned anywhere.

https://docs.microsoft.com/en-us/office365/servicedescriptions/office-365-platform-service-description/office-365-education

The https://products.office.com/en-gb/academic/compare-office-365-education-plans goes on about "Desktop versions of Office on up to 5 PCs or Macs per user" as well but this clearly isn't the case here.  Might be worth reaching out to however supplied the licences and just clarifying what was included or contacting Microsoft support and asking about the discrepancy.