Hi Sindhu,
Follow the below steps:
1. Create a line chart with the required dimension/measures.
2. Create a forecast.
View below solution to create a simple forecast.
https://www.edureka.co/community/44161/forecast-tableau?show=44161#q44161
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3. Not remove the forecast from the color shelf and add labels.
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4. Edit the forecast by right-clicking on the line and Go to forecast and then click on Forecast options, Edit the number of periods and prediction interval, and unselect Show prediction levels and click ok.
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Hope it helps.