WASHINGTON—U.S. Trade Representative Katherine Tai on Thursday declined to say if Taiwan would be invited to join the Biden administration’s Indo-Pacific economic plan, spurring Senate criticism that excluding the island would be a missed opportunity. Taiwan has voiced its desire to be a “full member” in the forthcoming Indo-Pacific Economic Framework (IPEF), which is one part of the administration’s effort to counter what it says is Beijing’s increasing economic and military coercion in the region. The administration says the still fledgling IPEF aims to be inclusive, but it has not publicly detailed any membership plans. IPEF is intended as a flexible economic framework that would align members on supply chain security, infrastructure, labor standards, green energy, and other issues. Tai, testifying before the Senate Finance Committee, called Taiwan an essential partner, but that no decisions had been made on membership. “On the point of Taiwan, we are in general in …