Position of 在 电脑上 in "他常练习在电脑上用汉语写东西"
In "他常在电脑上练习用汉语写东西",  the location '电脑上' is placed before the verb '练习'
  
  他常 (he often)
  
  在电脑上(on computer) -- location
  
  练习 practice -- verb
  
  用汉语写东西 -- relative clause


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  In "他常练习在电脑上用汉语写东西" ,  the location '电脑上' is placed before the verb phrase '用汉语写东西'
  
  他常(he often)
  
  练习 (verb)
  
  在电脑上用汉语写东西 -- relative clause


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  在电脑上 location 
  
  用汉语写东西 -- verb phrase (the verb is 写)


Both sentences mean "He often practices writing in Chinese on a computer". The difference is in the first sentence the location '在电脑上' is a relative phrase for the main verb '练习' (在电脑上练习) ; and in the second sentence, the location '在电脑上' is a relative phrase for the verb in the relative clause '用汉语写东西' (在电脑上用汉语写东西)