What is an ISO requirement & what is a recommendation? Can I choose not to follow recommendations?

ISO defines uses specific language to address this. (see https://www.iso.org/foreword-supplementary-information.html)
“shall” indicates a requirement
“should” indicates a recommendation
“may” is used to indicate that something is permitted
“can” is used to indicate that something is possible, for example, that an organization or individual is able to do something
Requirements and recommendations are defined as follows (ISO/IEC Directives, Part 2, 2021, 3.3.3-3.3.4) :
A requirement is an “expression, in the content of a document, that conveys objectively verifiable criteria to be fulfilled and from which no deviation is permitted if conformance with the document is to be claimed.”
A recommendation is an “expression, in the content of a document, that conveys a suggested possible choice or course of action deemed to be particularly suitable without necessarily mentioning or excluding others.”