SUMMARY
You cannot multiply by a decimal unless you put a zero in front of the decimal.  My Casio MS-80B can do this no problemo, so can my Android calculator, and the Google calculator

STEPS TO REPRODUCE
1. Type something like, "10x.5"
2. Press "="

OBSERVED RESULT

You get "Input error"

EXPECTED RESULT

You get "5".

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon 6.0
(available in About System)
KDE Plasma Version: 6.1.0
KDE Frameworks Version: 6.3.0 
Qt Version: 6.7.0

ADDITIONAL INFORMATION

This works if instead of "10x.5" you do "10x0.5". This also happens with division, addition, and subtraction operations. It is not obvious that you would need to put a zero before the decimal point