Does the phrase "i am ascending" in John 20:17 meant it an act in progress which could be completed before Jesus appears to those Jewish disciples who were hiding behind locked doors later that night in John 20
The verb here is Ἀναβαίνω (Anabainō) which is Present Indicative Active - 1st Person Singular. Thus, "I am ascending" is an accurate translation.
Jesus said this while standing in front of Mary with the instruction to tell the disciples, early Sunday morning. By the time she departed and travelled the short distance and reported to the disciples, Jesus certainly would be on His way, that is "ascending" to the Father. Thus, it is an accurate report of His actions.
Between the time that Jesus said this to Mary and when He appeared to the frightened disciples late that same day, He had ascended to the Father and returned to speak with them.
The present is used of the future, which He regards as immediately at hand. The message to the brethren is an assurance that the going to the Father, of which He had so often spoken to them, was about to be realised. The victory over death has been accomplished. This appearance on earth is an earnest of the return to heaven. "Unto My Father," He now says, "and your Father." It is a more emphatic expression than "our Father" would have been. "I ascend unto My Father. Because He is My Father, He is also your Father, and you are My brethren. My victory over death was the victory of man, whose nature has in Me conquered death. My ascension into heaven will be the ascension of human nature, which in Me goes to the Father."