Scripts are case-sensitive. Use if, not If.

Problematic code:

If true
Then
  echo "hello"
Fi

Correct code:

if true
then
  echo "hello"
fi

Rationale:

Shells are case sensitive and do not accept If or IF in place of lowercase if.

Exceptions

If you're aware of this and insist on naming a function WHILE, you can quote the name to prevent shellcheck from thinking you meant while. Or if you really want the names, add things like alias If=if IF=if to replace those keywords and ask shellcheck to ignore them.