Users almost always prefer simple sentences that use simple present or past tense and active voice. For example:
Registry Editor cannot create the subkey.
NOT: ...could not create...
Setup cannot start Program Manager.
NOT: Setup was unable to activate Program Manager.
Floppy disk sector ID field does not match floppy disk controller track address.
NOT: Mismatch between the floppy disk sector ID field and the
floppy disk controller track address.
CHKDSK encountered an error during...
NOT: An error was encountered during...
Cannot find %1.
NOT: Could not find...
Modem does not respond.
NOT: Modem not responding.
However, using active voice and a simple constructions is less important than making your message searchable. Use passive voice and more complex constructions if you need to in order to place search-relevant words at the beginning of the message. For example:
Log file size cannot be adjusted.
NOT: Cannot adjust the size of the log file.
NOT: Was unable to adjust the size of the log file.
Printing of %1 cannot resume.
NOT: Cannot resume printing %1.
NOT: Could not resume printing %1.