How to force a neighbor to comply with condo HOA rules regarding floor covering?

Based on your question, I assume that the condominium documents, meaning the declaration and/or bylaws, require that at least 80% of the floor area of the upper unit be covered by rugs or carpeting.  The codo documents typically will also include provisions about enforcement of rules, regulations and other requirements.  Typically the board of directors has enforcement power.  This issue must be brought before the condo board, which has a responsibility to enforce the rule regarding floor coverage.  The board must follow the provisions of the condo documents governing violations and enforcement.  Applicable state statutes will also be relevant regarding questions and issues that are not specifically covered in the condo documents.  Typically the board will give written notice to the offending unit owner and give that owner a specified period of time within which to cure the violation.  If they do not do so within the specified period of time, the board can impose fines as provided for in the condo documents.  If the unit owner still refuses to comply, the board can place a lien against their title, and bring a lawsuit to require compliance.  Condo documents or applicable state law usually provide that the board can take legal action to foreclose on a lien against a unit.  Frequently, unit owners who are confronted with this type of possible enforcement action and facing the risk of foreclosure will change their conduct and begin to comply with the rules and regulations.  This process would probably take several months.  If becomes necessary to file a lawsuit, the time involved could become much lengthier; possibly one year or more.  The cost of such action cannot be predicted based on the limited information available.