Turnitin will only accept files that can generate Similarity Reports. This includes:

files less than 100MB
files with a minimum of 20 words
file less than 800 pages
Microsoft Word (DOC and DOCX)
Corel WordPerfect
HTML
Adobe PostScript
Plain text (TXT)
Rich Text Format (RTF)
Portable Document Format (PDF)
OpenOffice (ODT)
Hangul (HWP and HWPX)
Powerpoint (PPTX)
Google Docs via Google Drive
Plain text files

Turnitin's file requirements for submissions provides more information.

Please keep in mind that Canvas does not support all of the file types that will generate a Turnitin Similarity Report and Turnitin will not generate a report for Apple Pages. Compatible text files that can be uploaded to Canvas include the following:

PDF - Portable Document Format
DOC, DOCX - Microsoft Word document
PPT, PPTX - PowerPoint document
XLS, XLSX - Excel document
Pages - Apple iWork Pages document
TXT - Plain text files
RTF - Rich Text Format
ODT, ODF - Open Document
HTML - HyperText Markup Language
Google Docs, Google Slides, Google Sheets (submitted via the Google Drive submission option)